usb: xhci-mtk: add support runtime PM

usb: xhci-mtk: add support runtime PM

A dedicated wakeup irq will be used to handle runtime suspend/resume,
we use dev_pm_set_dedicated_wake_irq API to take care of requesting
and attaching wakeup irq, then the suspend/resume framework will help
to enable/disable wakeup irq.

The runtime PM is default off since some platforms may not support it.
users can enable it via power/control (set "auto") in sysfs.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1618031406-15347-3-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>