usb: xhci-mtk: support option to disable usb2 ports

Add support to disable specific usb2 host ports, it's useful when
a usb2 port is disabled on some platforms, but enabled on others
for the same SoC.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1629189389-18779-4-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>