dt-bindings: Add bindings for USB3 phy on Allwinner H6

The new Allwinner H6 SoC contains a USB3 PHY that is wired to the
external USB3 pins of the SoC.

Add a device tree binding for the PHY.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>