dt-bindings: usb: dwc3: Add Amlogic G12A DWC3 Glue Bindings

Adds the bindings for the Amlogic G12A USB Glue HW.

The Amlogic G12A SoC Family embeds 2 USB Controllers :
- a DWC3 IP configured as Host for USB2 and USB3
- a DWC2 IP configured as Peripheral USB2 Only

A glue connects these both controllers to 2 USB2 PHYs,
and optionnally to an USB3+PCIE Combo PHY shared with the PCIE controller.

The Glue configures the UTMI 8bit interfaces for the USB2 PHYs, including
routing of the OTG PHY between the DWC3 and DWC2 controllers, and
setups the on-chip OTG mode selection for this PHY.

The PHYs phandles are passed  to the Glue node since the Glue controls the
interface with the PHY, not the DWC3 controller.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>