Documentation: bindings: add the Berlin SATA PHY

The Berlin SATA PHY drives the PHY related to the SATA interface. Add
the corresponding documentation.
Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/berlin-sata-phy.txt

+Berlin SATA PHY
+---------------
+
+Required properties:
+- compatible: should be "marvell,berlin2q-sata-phy"
+- address-cells: should be 1
+- size-cells: should be 0
+- phy-cells: from the generic PHY bindings, must be 1
+- reg: address and length of the register
+- clocks: reference to the clock entry
+
+Sub-nodes:
+Each PHY should be represented as a sub-node.
+
+Sub-nodes required properties:
+- reg: the PHY number
+
+Example:
+	sata_phy: phy@f7e900a0 {
+		compatible = "marvell,berlin2q-sata-phy";
+		reg = <0xf7e900a0 0x200>;
+		clocks = <&chip CLKID_SATA>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		#phy-cells = <1>;
+
+		sata-phy@0 {
+			reg = <0>;
+		};
+
+		sata-phy@1 {
+			reg = <1>;
+		};
+	};