dt-bindings: Add Cygnus PCIe PHY binding doc

Add DT binding document for Broadcom Cygnus PCIe PHYs
Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Arun Parameswaran <aparames@broadcom.com>
Reviewed-by: JD (Jiandong) Zheng <jdzheng@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/brcm,cygnus-pcie-phy.txt

+Broadcom Cygnus PCIe PHY
+
+Required properties:
+- compatible: must be "brcm,cygnus-pcie-phy"
+- reg: base address and length of the PCIe PHY block
+- #address-cells: must be 1
+- #size-cells: must be 0
+
+Each PCIe PHY should be represented by a child node
+
+Required properties For the child node:
+- reg: the PHY ID
+0 - PCIe RC 0
+1 - PCIe RC 1
+- #phy-cells: must be 0
+
+Example:
+	pcie_phy: phy@0301d0a0 {
+		compatible = "brcm,cygnus-pcie-phy";
+		reg = <0x0301d0a0 0x14>;
+
+		pcie0_phy: phy@0 {
+			reg = <0>;
+			#phy-cells = <0>;
+		};
+
+		pcie1_phy: phy@1 {
+			reg = <1>;
+			#phy-cells = <0>;
+		};
+	};
+
+	/* users of the PCIe phy */
+
+	pcie0: pcie@18012000 {
+		...
+		...
+		phys = <&pcie0_phy>;
+		phy-names = "pcie-phy";
+	};
+
+	pcie1: pcie@18013000 {
+		...
+		...
+		phys = <pcie1_phy>;
+		phy-names = "pcie-phy";
+	};