mach-meson: g12a: add DWC2 peripheral mode support

Adds support for Amlogic G12A USB Device mode.

The DWC2 Controller behind the Glue can be connected to an OTG
capable PHY. The Glue setups the PHY mode.

This patch implements Device mode support by adding a board_usb_init/cleanup
setting up the DWC2 controller and switch the OTG capable port to Device
before starting the DWC2 controller in Device mode.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>