dt-bindings: dmaengine: usb-dmac: Add binding for r8a7744

This patch adds binding for r8a7744 (RZ/G1N).

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vkoul@kernel.org>