MediaTek xHCI

The device node for USB3 host controller on MediaTek SoCs.

Required properties:
 - compatible : should be "mediatek,mtk-xhci"
 - reg : specifies physical base address and size of the registers
 - reg-names: should be "mac" for xHCI MAC and "ippc" for IP port control
 - power-domains : a phandle to USB power domain node to control USB's
	MTCMOS
 - vusb33-supply : regulator of USB avdd3.3v

 - clocks : a list of phandle + clock-specifier pairs, one for each
	entry in clock-names
 - clock-names : must contain
	"sys_ck": controller clock used by normal mode,
	the following ones are optional:
	"ref_ck": reference clock used by low power mode etc,
	"mcu_ck": mcu_bus clock for register access,
	"dma_ck": dma_bus clock for data transfer by DMA,
	"xhci_ck": controller clock

 - phys : list of all the USB PHYs on this HCD
 - phy-names: name specifier for the USB PHY

Optional properties:
 - vbus-supply : reference to the VBUS regulator;

Example:
xhci: usb@1a0c0000 {
	compatible = "mediatek,mt7629-xhci", "mediatek,mtk-xhci";
	reg = <0x1a0c0000 0x1000>, <0x1a0c3e00 0x0100>;
	reg-names = "mac", "ippc";
	power-domains = <&scpsys MT7629_POWER_DOMAIN_HIF1>;
	clocks = <&ssusbsys CLK_SSUSB_SYS_EN>, <&ssusbsys CLK_SSUSB_REF_EN>,
		 <&ssusbsys CLK_SSUSB_MCU_EN>, <&ssusbsys CLK_SSUSB_DMA_EN>;
	clock-names = "sys_ck", "ref_ck", "mcu_ck", "dma_ck";
	phys = <&u2port0 PHY_TYPE_USB2>, <&u3port0 PHY_TYPE_USB3>;
	status = "disabled";
};