ARM: mmp: add a pxa-usb-phy device

This is to replace the USB PHY initialization code (pxa_usb_phy_init(),
pxa_usb_phy_deinit()) with a proper PHY driver.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Olof Johansson <olof@lixom.net>