usb: mtu3: support option to disable usb3 ports

Add support to disable specific usb3 ports, it's useful when
usb3 phy is shared with PCIe or SATA, because we should disable
the corresponding usb3 port if the phy is used by PCIe or SATA.

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