dt-bindings: usb: add support for dwc3 controller on Amlogic Meson GX

Amlogic Meson GX SoCs (GXL and AXG) come with a (host-only) dwc3 USB
controller. This requires a clock to be enabled and a reset line to be
pulsed to get the hardware into a known state.
Add the documentation for this IP block, similar to "qcom,dwc3.txt".

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>