dt-bindings: usb: add bindings doc for HiSilicon STB xHCI host controller

HiSilicon STB xHCI

The device node for HiSilicon STB xHCI host controller

Required properties:

 - compatible: should be "hisilicon,hi3798cv200-xhci"

 - reg: specifies physical base address and size of the registers

 - interrupts : interrupt used by the controller

 - clocks: a list of phandle + clock-specifier pairs, one for each

	entry in clock-names

 - clock-names: must contain

	"bus": for bus clock

	"utmi": for utmi clock

	"pipe": for pipe clock

	"suspend": for suspend clock

 - resets: a list of phandle and reset specifier pairs as listed in

	reset-names property.

 - reset-names: must contain

	"soft": for soft reset

 - phys: a list of phandle + phy specifier pairs

 - phy-names: must contain at least one of following:

	"inno": for inno phy

	"combo": for combo phy

Optional properties:

  - usb2-lpm-disable: indicate if we don't want to enable USB2 HW LPM

  - usb3-lpm-capable: determines if platform is USB3 LPM capable

  - imod-interval-ns: default interrupt moderation interval is 40000ns

Example:

xhci0: xchi@f98a0000 {

	compatible = "hisilicon,hi3798cv200-xhci";

	reg = <0xf98a0000 0x10000>;

	interrupts = <GIC_SPI 69 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&crg HISTB_USB3_BUS_CLK>,

		 <&crg HISTB_USB3_UTMI_CLK>,

		 <&crg HISTB_USB3_PIPE_CLK>,

		 <&crg HISTB_USB3_SUSPEND_CLK>;

	clock-names = "bus", "utmi", "pipe", "suspend";

	resets = <&crg 0xb0 12>;

	reset-names = "soft";

	phys = <&usb2_phy1_port1 0>, <&combphy0 PHY_TYPE_USB3>;

	phy-names = "inno", "combo";

};