Documentation: bindings: add dt documentation for Rockchip eMMC PHY

This patch adds a binding that describes the Rockchip eMMC PHYs
found on Rockchip SoCs eMMC interface.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/rockchip-emmc-phy.txt

+Rockchip EMMC PHY
+-----------------------
+
+Required properties:
+ - compatible: rockchip,rk3399-emmc-phy
+ - rockchip,grf : phandle to the syscon managing the "general
+   register files"
+ - #phy-cells: must be 0
+ - reg: PHY configure reg address offset in "general
+   register files"
+
+Example:
+
+emmcphy: phy {
+	compatible = "rockchip,rk3399-emmc-phy";
+	rockchip,grf = <&grf>;
+	reg = <0xf780>;
+	#phy-cells = <0>;
+};