usb: otg: OMAP4430: Powerdown the internal PHY when USB is disabled

Powerdown the internal UTMI PHY when USB is not enabled. This will
allow the OMAP core domain to transition to retention and offmode.

Signed-off-by: Hema HK <hemahk@ti.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>