usb: mtu3: add optional mcu and dma bus clocks

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

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>