dt-bindings: dmaengine: usb-dmac: Add binding for r8a774c0

This patch adds bindings for the r8a774c0 (RZ/G2E).

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>