phy: phy-pxa-usb: add a new driver

Turned from arch/arm/mach-mmp/devices.c into a proper PHY driver, so
that in can be instantiated from a DT.

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>