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

Add a new optional clock xhci_ck
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1566542425-20082-1-git-send-email-chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/mediatek,mtk-xhci.txt

@@ -30,7 +30,8 @@ Required properties:
 	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
 
@@ -100,7 +101,7 @@ Required properties:
  - 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;