dt-bindings: phy: Document cadence Sierra PHY bindings

Add DT binding documentation for Sierra PHY.  The PHY supports
a number of different protocols, including PCIe and USB.

The PHY lanes may be configured as single or multi-lane links.
Each link is treated as a separate sub-node.  For example, if
there are 4 lanes in total the first 2 might be configured as
a multi-lane PCIe link while the other two are single lane
USB links, and in this case there would be 3 sub-nodes.

There are two resets for the PHY block (one for APB register
access, one for the PHY link) and separate resets for each
link.  For multi-lane links, the reset corresponds to the
reset line on the master lane, the resets on other lanes
have no effect.
Signed-off-by: Alan Douglas <adouglas@cadence.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/phy/phy-cadence-sierra.txt

+Cadence Sierra PHY
+-----------------------
+
+Required properties:
+- compatible:	cdns,sierra-phy-t0
+- clocks:	Must contain an entry in clock-names.
+		See ../clocks/clock-bindings.txt for details.
+- clock-names:	Must be "phy_clk"
+- resets:	Must contain an entry for each in reset-names.
+		See ../reset/reset.txt for details.
+- reset-names:	Must include "sierra_reset" and "sierra_apb".
+		"sierra_reset" must control the reset line to the PHY.
+		"sierra_apb" must control the reset line to the APB PHY
+		interface.
+- reg:		register range for the PHY.
+- #address-cells: Must be 1
+- #size-cells:	Must be 0
+
+Optional properties:
+- cdns,autoconf:	A boolean property whose presence indicates that the
+			PHY registers will be configured by hardware. If not
+			present, all sub-node optional properties must be
+			provided.
+
+Sub-nodes:
+  Each group of PHY lanes with a single master lane should be represented as
+  a sub-node. Note that the actual configuration of each lane is determined by
+  hardware strapping, and must match the configuration specified here.
+
+Sub-node required properties:
+- #phy-cells:	Generic PHY binding; must be 0.
+- reg:		The master lane number.  This is the lowest numbered lane
+		in the lane group.
+- resets:	Must contain one entry which controls the reset line for the
+		master lane of the sub-node.
+		See ../reset/reset.txt for details.
+
+Sub-node optional properties:
+- cdns,num-lanes:	Number of lanes in this group.  From 1 to 4.  The
+			group is made up of consecutive lanes.
+- cdns,phy-type:	Can be PHY_TYPE_PCIE or PHY_TYPE_USB3, depending on
+			configuration of lanes.
+
+Example:
+	pcie_phy4: pcie-phy@fd240000 {
+		compatible = "cdns,sierra-phy-t0";
+		reg = <0x0 0xfd240000 0x0 0x40000>;
+		resets = <&phyrst 0>, <&phyrst 1>;
+		reset-names = "sierra_reset", "sierra_apb";
+		clocks = <&phyclock>;
+		clock-names = "phy_clk";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		pcie0_phy0: pcie-phy@0 {
+				reg = <0>;
+				resets = <&phyrst 2>;
+				cdns,num-lanes = <2>;
+				#phy-cells = <0>;
+				cdns,phy-type = <PHY_TYPE_PCIE>;
+		};
+		pcie0_phy1: pcie-phy@2 {
+				reg = <2>;
+				resets = <&phyrst 4>;
+				cdns,num-lanes = <1>;
+				#phy-cells = <0>;
+				cdns,phy-type = <PHY_TYPE_PCIE>;
+		};