usb: phy: phy-mxs-usb: add power down and disable wakeup for .shutdown

When we shut down the PHY, we need to power down all PHY's functions
as well as disable wakeup, it is the opposite operation we do at .init.

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