dt-bindings: usb: mtk-xhci: add optional assigned clock properties

Add optional property "assigned-clock" and "assigned-clock-parents"
used by mt7629.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20210201070016.41721-6-chunfeng.yun@mediatek.com


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