dt-bindings: usb: dwc2: add support for APM82181 SoCs USB OTG HS and FS

adds the specific compatible string for the DWC2 IP found in the APM82181
SoCs. The APM82181's USB-OTG seems like it was taken from its direct
predecessor: the PPC460EX (canyonlands).

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>