dt-bindings: Add Cygnus PCIe PHY binding doc

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";

	};