usb: gadget: udc: renesas_usb3: Add r8a774a1 support

Document RZ/G2M (R8A774A1) SoC bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>