dt-bindings: usb: mtu3: add two optional clocks

Add optional clock 'xhci_ck' and 'frmcnt_ck';

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20230417025203.18097-6-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>