dt-bindings: usb: mtk-xhci: remove dummy clocks and add optional ones

Remove dummy clocks for usb wakeup and add optional ones for
MCU_BUS_CK and DMA_BUS_CK.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>