dt-bindings: mailbox: Document Hi6220 mailbox driver

Document DT binding for Hisilicon Hi6220 mailbox driver.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/hisilicon,hi6220-mailbox.txt

+Hisilicon Hi6220 Mailbox Driver
+===============================
+
+Hisilicon Hi6220 mailbox supports up to 32 channels. Each channel
+is unidirectional with a maximum message size of 8 words. I/O is
+performed using register access (there is no DMA) and the cell
+raises an interrupt when messages are received.
+
+Mailbox Device Node:
+====================
+
+Required properties:
+--------------------
+- compatible:		Shall be "hisilicon,hi6220-mbox"
+- reg:			Contains the mailbox register address range (base
+			address and length); the first item is for IPC
+			registers, the second item is shared buffer for
+			slots.
+- #mbox-cells:		Common mailbox binding property to identify the number
+			of cells required for the mailbox specifier. Must be 3.
+			<&phandle slot_id dst_irq ack_irq>
+			  phandle: Label name of mailbox controller
+			  slot_id: Slot id used either for TX or RX
+			  dst_irq: IRQ identifier index number which used by MCU
+			  ack_irq: IRQ identifier index number with generating a
+			           TX/RX interrupt to application processor,
+				   mailbox driver uses it to acknowledge interrupt
+- interrupts:		Contains the interrupt information for the mailbox
+			device. The format is dependent on which interrupt
+			controller the SoCs use.
+
+Optional Properties:
+--------------------
+- hi6220,mbox-tx-noirq: Property of MCU firmware's feature, so mailbox driver
+			use this flag to ask MCU to enable "automatic idle
+			flag" mode or IRQ generated mode to acknowledge a TX
+			completion.
+
+Example:
+--------
+
+	mailbox: mailbox@f7510000 {
+		compatible = "hisilicon,hi6220-mbox";
+		reg = <0x0 0xf7510000 0x0 0x1000>, /* IPC_S */
+		      <0x0 0x06dff800 0x0 0x0800>; /* Mailbox */
+		interrupt-parent = <&gic>;
+		interrupts = <GIC_SPI 94 IRQ_TYPE_LEVEL_HIGH>;
+		#mbox-cells = <3>;
+	};
+
+
+Mailbox client
+===============
+
+Required properties:
+--------------------
+- compatible:		Many (See the client docs).
+- mboxes:		Standard property to specify a Mailbox (See ./mailbox.txt)
+			Cells must match 'mbox-cells' (See Mailbox Device Node above).
+
+Optional Properties:
+--------------------
+- mbox-names:		Name given to channels seen in the 'mboxes' property.
+
+Example:
+--------
+
+	stub_clock: stub_clock {
+		compatible = "hisilicon,hi6220-stub-clk";
+		hisilicon,hi6220-clk-sram = <&sram>;
+		#clock-cells = <1>;
+		mbox-names = "mbox-tx", "mbox-rx";
+		mboxes = <&mailbox 1 0 11>, <&mailbox 0 1 10>;
+	};