dt-bindings: usb: dwc2: add compatible property for rk3368 usb

A test with the command below gives for example this error:

arch/arm64/boot/dts/rockchip/rk3368-evb-act8846.dt.yaml: usb@ff580000:
compatible:
['rockchip,rk3368-usb', 'rockchip,rk3066-usb', 'snps,dwc2']
is not valid under any of the given schemas

The compatible property for rk3368 dwc2 usb was somehow never added to
the documention. Fix this error by adding
'rockchip,rk3368-usb', 'rockchip,rk3066-usb', 'snps,dwc2'
to dwc2.yaml.

make ARCH=arm64 dtbs_check
DT_SCHEMA_FILES=Documentation/devicetree/bindings/usb/dwc2.yaml

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>