dt-bindings: phy: Add Amlogic G12A USB3+PCIE Combo PHY Bindings

* Amlogic G12A USB3 + PCIE Combo PHY binding

Required properties:

- compatible:	Should be "amlogic,meson-g12a-usb3-pcie-phy"

- #phys-cells:	must be 1. The cell number is used to select the phy mode

  as defined in <dt-bindings/phy/phy.h> between PHY_TYPE_USB3 and PHY_TYPE_PCIE

- reg:		The base address and length of the registers

- clocks:	a phandle to the 100MHz reference clock of this PHY

- clock-names:	must be "ref_clk"

- resets:	phandle to the reset lines for the PHY control

- reset-names:	must be "phy"

Example:

	usb3_pcie_phy: phy@46000 {

		compatible = "amlogic,g12a-usb3-pcie-phy";

		reg = <0x0 0x46000 0x0 0x2000>;

		clocks = <&clkc CLKID_PCIE_PLL>;

		clock-names = "ref_clk";

		resets = <&reset RESET_PCIE_PHY>;

		reset-names = "phy";

		#phy-cells = <1>;

	};