Describe Audio-DMAC in the R8A7794 device tree.

This patch is loosely based on the commit 8994fff6 ("ARM: shmobile:
r8a7791: Add Audio DMAC devices to DT").
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>