dt-bindings: usb: mtk-xhci: add an optional xhci_ck clock

	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

	"dma_ck": dma_bus clock for data transfer by DMA,

	"xhci_ck": controller clock

 - phys : see usb-hcd.txt in the current directory

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

	entry in clock-names

 - clock-names : must contain "sys_ck", and the following ones are optional:

	"ref_ck", "mcu_ck" and "dma_ck"

	"ref_ck", "mcu_ck" and "dma_ck", "xhci_ck"

Optional properties:

 - vbus-supply : reference to the VBUS regulator;