usb: phy: mxs: Add system suspend/resume API

We need this to keep PHY's power on or off during the system
suspend mode. If we need to enable USB wakeup, then we
must keep PHY's power being on during the system suspend mode.
Otherwise, we need to keep PHY's power being off to save power.

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