phy: ralink-usb: add driver for Mediatek/Ralink

Add a driver to setup the USB phy on Mediatek/Ralink SoCs.
The driver sets up power and host mode, but also needs to
configure PHY registers for the MT7628 and MT7688.

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>