dt-bindings: phy: Add DT binding for Mixel LVDS PHY(LVDS/MIPI DSI combo)

This patch adds device tree binding for Mixel LVDS PHY(LVDS/MIPI DSI combo),
as found in i.MX8qxp SoC.

Signed-off-by: Liu Ying <victor.liu@nxp.com>

Documentation/devicetree/bindings/phy/phy-mixel-lvds-combo.txt

@@ -0,0 +1,19 @@
+Mixel LVDS combo PHY
+
+Required properties:
+- compatible: must be "mixel,lvds-combo-phy".
+- reg: offset and length of the register block.
+- #phy-cells: see phy-bindings.txt in the same directory, must be <0>.
+- clocks: clock phandle and specifier pair.
+- clock-names: string, clock input name, must be "phy".
+- power-domains: phandle pointing to power domain.
+
+Example:
+	ldb_phy@56221000 {
+		compatible = "mixel,lvds-combo-phy";
+		reg = <0x0 0x56221000 0x0 0x100>, <0x0 0x56228000 0x0 0x1000>;
+		#phy-cells = <0>;
+		clocks = <&clk IMX_LVDS0_PHY_CLK>;
+		clock-names = "phy";
+		power-domains = <&pd IMX_SC_R_LVDS_0>;
+	};