dt-bindings: add bindings doc for hi3798cv200 combphy

HiSilicon STB PCIE/SATA/USB3 PHY

Required properties:

- compatible: Should be "hisilicon,hi3798cv200-combphy"

- reg: Should be the address space for COMBPHY configuration and state

  registers in peripheral controller, e.g. PERI_COMBPHY0_CFG and

  PERI_COMBPHY0_STATE for COMBPHY0 Hi3798CV200 SoC.

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

  as defined in <dt-bindings/phy/phy.h>.

- clocks: The phandle to clock provider and clock specifier pair.

- resets: The phandle to reset controller and reset specifier pair.

Refer to phy/phy-bindings.txt for the generic PHY binding properties.

Optional properties:

- hisilicon,fixed-mode: If the phy device doesn't support mode select

  but a fixed mode setting, the property should be present to specify

  the particular mode.

- hisilicon,mode-select-bits: If the phy device support mode select,

  this property should be present to specify the register bits in

  peripheral controller, as a 3 integers tuple:

  <register_offset bit_shift bit_mask>.

Notes:

- Between hisilicon,fixed-mode and hisilicon,mode-select-bits, one and only

  one of them should be present.

- The device node should be a child of peripheral controller that contains

  COMBPHY configuration/state and PERI_CTRL register used to select PHY mode.

  Refer to arm/hisilicon/hisilicon.txt for the parent peripheral controller

  bindings.

Examples:

perictrl: peripheral-controller@8a20000 {

	compatible = "hisilicon,hi3798cv200-perictrl", "syscon",

		     "simple-mfd";

	reg = <0x8a20000 0x1000>;

	#address-cells = <1>;

	#size-cells = <1>;

	ranges = <0x0 0x8a20000 0x1000>;

	combphy0: phy@850 {

		compatible = "hisilicon,hi3798cv200-combphy";

		reg = <0x850 0x8>;

		#phy-cells = <1>;

		clocks = <&crg HISTB_COMBPHY0_CLK>;

		resets = <&crg 0x188 4>;

		hisilicon,fixed-mode = <PHY_TYPE_USB3>;

	};

	combphy1: phy@858 {

		compatible = "hisilicon,hi3798cv200-combphy";

		reg = <0x858 0x8>;

		#phy-cells = <1>;

		clocks = <&crg HISTB_COMBPHY1_CLK>;

		resets = <&crg 0x188 12>;

		hisilicon,mode-select-bits = <0x0008 11 (0x3 << 11)>;

	};

};