dt-bindings: usb: mtk-xhci: add an optional frame count clock

Add optional clock 'frmcnt_ck' used on 4nm or advanced process SoC
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20230407062406.12575-1-chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -77,6 +77,7 @@ properties:
       - description: Mcu bus clock for register access
       - description: DMA bus clock for data transfer
       - description: controller clock
+      - description: frame count clock
 
   clock-names:
     minItems: 1
@@ -86,6 +87,7 @@ properties:
       - const: mcu_ck
       - const: dma_ck
       - const: xhci_ck
+      - const: frmcnt_ck
 
   phys:
     description: