dt-bindings: arm: fsl: add scu binding doc

The System Controller Firmware (SCFW) is a low-level system function
which runs on a dedicated Cortex-M core to provide power, clock, and
resource management. It exists on some i.MX8 processors. e.g. i.MX8QM
(QM, QP), and i.MX8QX (QXP, DX).

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/freescale/fsl,scu.txt

+NXP i.MX System Controller Firmware (SCFW)
+--------------------------------------------------------------------
+
+The System Controller Firmware (SCFW) is a low-level system function
+which runs on a dedicated Cortex-M core to provide power, clock, and
+resource management. It exists on some i.MX8 processors. e.g. i.MX8QM
+(QM, QP), and i.MX8QX (QXP, DX).
+
+The AP communicates with the SC using a multi-ported MU module found
+in the LSIO subsystem. The current definition of this MU module provides
+5 remote AP connections to the SC to support up to 5 execution environments
+(TZ, HV, standard Linux, etc.). The SC side of this MU module interfaces
+with the LSIO DSC IP bus. The SC firmware will communicate with this MU
+using the MSI bus.
+
+System Controller Device Node:
+============================================================
+
+The scu node with the following properties shall be under the /firmware/ node.
+
+Required properties:
+-------------------
+- compatible:	should be "fsl,imx-scu".
+- mbox-names:	should include "tx0", "tx1", "tx2", "tx3",
+			       "rx0", "rx1", "rx2", "rx3".
+- mboxes:	List of phandle of 4 MU channels for tx and 4 MU channels
+		for rx. All 8 MU channels must be in the same MU instance.
+		Cross instances are not allowed. The MU instance can only
+		be one of LSIO MU0~M4 for imx8qxp and imx8qm. Users need
+		to make sure use the one which is not conflict with other
+		execution environments. e.g. ATF.
+		Note:
+		Channel 0 must be "tx0" or "rx0".
+		Channel 1 must be "tx1" or "rx1".
+		Channel 2 must be "tx2" or "rx2".
+		Channel 3 must be "tx3" or "rx3".
+		e.g.
+		mboxes = <&lsio_mu1 0 0
+			  &lsio_mu1 0 1
+			  &lsio_mu1 0 2
+			  &lsio_mu1 0 3
+			  &lsio_mu1 1 0
+			  &lsio_mu1 1 1
+			  &lsio_mu1 1 2
+			  &lsio_mu1 1 3>;
+		See Documentation/devicetree/bindings/mailbox/fsl,mu.txt
+		for detailed mailbox binding.
+
+i.MX SCU Client Device Node:
+============================================================
+
+Client nodes are maintained as children of the relevant IMX-SCU device node.
+
+Power domain bindings based on SCU Message Protocol
+------------------------------------------------------------
+
+This binding for the SCU power domain providers uses the generic power
+domain binding[2].
+
+Required properties:
+- compatible:		Should be "fsl,scu-pd".
+- #address-cells:	Should be 1.
+- #size-cells:		Should be 0.
+
+Required properties for power domain sub nodes:
+- #power-domain-cells:	Must be 0.
+
+Optional Properties:
+- reg:			Resource ID of this power domain.
+			No exist means uncontrollable by user.
+			See detailed Resource ID list from:
+			include/dt-bindings/power/imx-rsrc.h
+- power-domains:	phandle pointing to the parent power domain.
+
+Clock bindings based on SCU Message Protocol
+------------------------------------------------------------
+
+This binding uses the common clock binding[1].
+
+Required properties:
+- compatible:		Should be "fsl,imx8qxp-clock".
+- #clock-cells:		Should be 1. Contains the Clock ID value.
+- clocks:		List of clock specifiers, must contain an entry for
+			each required entry in clock-names
+- clock-names:		Should include entries "xtal_32KHz", "xtal_24MHz"
+
+The clock consumer should specify the desired clock by having the clock
+ID in its "clocks" phandle cell.
+
+See the full list of clock IDs from:
+include/dt-bindings/clock/imx8qxp-clock.h
+
+Pinctrl bindings based on SCU Message Protocol
+------------------------------------------------------------
+
+This binding uses the i.MX common pinctrl binding[3].
+
+Required properties:
+- compatible:		Should be "fsl,imx8qxp-iomuxc".
+
+Required properties for Pinctrl sub nodes:
+- fsl,pins:		Each entry consists of 3 integers which represents
+			the mux and config setting for one pin. The first 2
+			integers <pin_id mux_mode> are specified using a
+			PIN_FUNC_ID macro, which can be found in
+			<dt-bindings/pinctrl/pads-imx8qxp.h>.
+			The last integer CONFIG is the pad setting value like
+			pull-up on this pin.
+
+			Please refer to i.MX8QXP Reference Manual for detailed
+			CONFIG settings.
+
+[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
+[2] Documentation/devicetree/bindings/power/power_domain.txt
+[3] Documentation/devicetree/bindings/pinctrl/fsl,imx-pinctrl.txt
+
+Example (imx8qxp):
+-------------
+lsio_mu1: mailbox@5d1c0000 {
+	...
+	#mbox-cells = <2>;
+};
+
+firmware {
+	scu {
+		compatible = "fsl,imx-scu";
+		mbox-names = "tx0", "tx1", "tx2", "tx3",
+			     "rx0", "rx1", "rx2", "rx3";
+		mboxes = <&lsio_mu1 0 0
+			  &lsio_mu1 0 1
+			  &lsio_mu1 0 2
+			  &lsio_mu1 0 3
+			  &lsio_mu1 1 0
+			  &lsio_mu1 1 1
+			  &lsio_mu1 1 2
+			  &lsio_mu1 1 3>;
+
+		clk: clk {
+			compatible = "fsl,imx8qxp-clk";
+			#clock-cells = <1>;
+		};
+
+		iomuxc {
+			compatible = "fsl,imx8qxp-iomuxc";
+
+			pinctrl_lpuart0: lpuart0grp {
+				fsl,pins = <
+					SC_P_UART0_RX_ADMA_UART0_RX	0x06000020
+					SC_P_UART0_TX_ADMA_UART0_TX	0x06000020
+				>;
+			};
+			...
+		};
+
+		imx8qx-pm {
+			compatible = "fsl,scu-pd";
+			#address-cells = <1>;
+			#size-cells = <0>;
+
+			pd_dma: dma-power-domain {
+				#power-domain-cells = <0>;
+
+				pd_dma_lpuart0: dma-lpuart0@57 {
+					reg = <SC_R_UART_0>;
+					#power-domain-cells = <0>;
+					power-domains = <&pd_dma>;
+				};
+				...
+			};
+			...
+		};
+	};
+};
+
+serial@5a060000 {
+	...
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_lpuart0>;
+	clocks = <&clk IMX8QXP_UART0_CLK>,
+		 <&clk IMX8QXP_UART0_IPG_CLK>;
+	clock-names = "per", "ipg";
+	power-domains = <&pd_dma_lpuart0>;
+};