usb: phy: mxs: Add implementation of set_wakeup

When we need the PHY can be waken up by external signals,
we can call this API. Besides, we call mxs_phy_disconnect_line
at this API to close the connection between USB PHY and
controller, after that, the line state from controller is SE0.
Once the PHY is out of power, without calling mxs_phy_disconnect_line,
there are unknown wakeups due to dp/dm floating at device mode.

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