Documentation: dt: Add new compatible for the A31 USB Phy

Document the freshly introduced compatible for the USB phy in use in the
Allwinner A31 SoC.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

diff --cc Documentation/devicetree/bindings/phy/sun4i-usb-phy.txt
Simple merge