dt-bindings: phy: Add binding doc for Stingray PCIe PHY

Broadcom Stingray PCIe PHY

Required properties:

- compatible: must be "brcm,sr-pcie-phy"

- reg: base address and length of the PCIe SS register space

- brcm,sr-cdru: phandle to the CDRU syscon node

- brcm,sr-mhb: phandle to the MHB syscon node

- #phy-cells: Must be 1, denotes the PHY index

For PAXB based root complex, one can have a configuration of up to 8 PHYs

PHY index goes from 0 to 7

For the internal PAXC based root complex, PHY index is always 8

Example:

	mhb: syscon@60401000 {

		compatible = "brcm,sr-mhb", "syscon";

		reg = <0 0x60401000 0 0x38c>;

	};

	cdru: syscon@6641d000 {

		compatible = "brcm,sr-cdru", "syscon";

		reg = <0 0x6641d000 0 0x400>;

	};

	pcie_phy: phy@40000000 {

		compatible = "brcm,sr-pcie-phy";

		reg = <0 0x40000000 0 0x800>;

		brcm,sr-cdru = <&cdru>;

		brcm,sr-mhb = <&mhb>;

		#phy-cells = <1>;

	};

	/* users of the PCIe PHY */

	pcie0: pcie@48000000 {

		...

		...

		phys = <&pcie_phy 0>;

		phy-names = "pcie-phy";

	};