usb: gadget: udc: renesas_usb3: Add bindings for r8a774c0

Document RZ/G2E (R8A774C0) SoC bindings.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>