dt-bindings: phy: Add document for phy-mtk-ufs

MediaTek Universal Flash Storage (UFS) M-PHY binding

--------------------------------------------------------

UFS M-PHY nodes are defined to describe on-chip UFS M-PHY hardware macro.

Each UFS M-PHY node should have its own node.

To bind UFS M-PHY with UFS host controller, the controller node should

contain a phandle reference to UFS M-PHY node.

Required properties for UFS M-PHY nodes:

- compatible         : Compatible list, contains the following controller:

                       "mediatek,mt8183-ufsphy" for ufs phy

                       persent on MT81xx chipsets.

- reg                : Address and length of the UFS M-PHY register set.

- #phy-cells         : This property shall be set to 0.

- clocks             : List of phandle and clock specifier pairs.

- clock-names        : List of clock input name strings sorted in the same

                       order as the clocks property. Following clocks are

                       mandatory.

                       "unipro": Unipro core control clock.

                       "mp": M-PHY core control clock.

Example:

	ufsphy: phy@11fa0000 {

		compatible = "mediatek,mt8183-ufsphy";

		reg = <0 0x11fa0000 0 0xc000>;

		#phy-cells = <0>;

		clocks = <&infracfg_ao INFRACFG_AO_UNIPRO_SCK_CG>,

			 <&infracfg_ao INFRACFG_AO_UFS_MP_SAP_BCLK_CG>;

		clock-names = "unipro", "mp";

	};

	ufshci@11270000 {

		...

		phys = <&ufsphy>;

	};