usb: xhci-mtk: add optional mcu and dma bus clocks

There are mcu_bus and dma_bus clocks needed to be controlled by
driver on some SoCs, so add them as optional ones

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>