usb: phy: Add set_wakeup API

This API is used to set wakeup enable at PHY registers, in that
case, the PHY can be waken up from suspend due to external events,
like vbus change, dp/dm change and id change.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>