ARM: shmobile: r8a7793: Add DMAC devices to DT

Instantiate the two system DMA controllers in the r8a7793 device tree.

Based on similar work for the r8a7793 by Laurent Pinchart.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>