dt-bindings: phy: add binding for Allwinner H6 USB2 PHY

The USB2.0 PHY on Allwinner H6 is similar to the ones on the ones on
older SoCs, but with holes in PHY number (USB1 and USB2 are missing, in
which USB1 is a USB3 PHY).

Add binding for the PHY.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>