dt-bindings: phy-pxa-usb: add bindings

This is the PHY chip for USB OTG on PXA platforms.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>