dt-bindings: phy: Add bindings for ralink-usb PHY

Add a binding for the USB phy on Mediatek/Ralink SoCs.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>