Documentation: devicetree: add Broadcom SATA PHY binding

* Broadcom SATA3 PHY for STB

Required properties:

- compatible: should be one or more of

     "brcm,bcm7445-sata-phy"

     "brcm,phy-sata3"

- address-cells: should be 1

- size-cells: should be 0

- reg: register range for the PHY PCB interface

- reg-names: should be "phy"

Sub-nodes:

  Each port's PHY should be represented as a sub-node.

Sub-nodes required properties:

- reg: the PHY number

- phy-cells: generic PHY binding; must be 0

Optional:

- brcm,enable-ssc: use spread spectrum clocking (SSC) on this port

Example:

	sata-phy@f0458100 {

		compatible = "brcm,bcm7445-sata-phy", "brcm,phy-sata3";

		reg = <0xf0458100 0x1e00>, <0xf045804c 0x10>;

		reg-names = "phy";

		#address-cells = <1>;

		#size-cells = <0>;

		sata-phy@0 {

			reg = <0>;

			#phy-cells = <0>;

		};

		sata-phy@1 {

			reg = <1>;

			#phy-cells = <0>;

		};

	};