usb: dwc3: Add Meson G12A USB Glue

Adds support for Amlogic G12A USB Control 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.

This driver sets the OTG capable port as Host mode by default,
the switch to Device mode is to be done in a separate patch.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Marek Vasut <marex@denx.de>