phy: usb: sunxi: Introduce Allwinner A31 USB PHY support

The USB phy controller in the A31 differs mostly from the older controllers
because it has a clock dedicated for each phy, while the older ones were having
a single clock for all the phys.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>