dt-bindings: phy: Add Amlogic G12A USB2 PHY Bindings

Add the Amlogic G12A Family USB2 OTG PHY Bindings

The PHY can work in host or peripheral modes depending on it's position.
Configuration of the mode is part of the USBCTRL registers which are
outside of the PHY registers.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>