ARM: shmobile: r8a7791 dtsi: add SYS-DMAC clocks

Add clocks for the SYS-DMAC0 and SYS-DMAC1 hardware blocks.

Cfr. the r8a7790 version by Ben Dooks.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>