arm64: dts: r8a7795: Add SoC-specific compatible property to audio-dmac nodes

The audio-dmac nodes used the generic compatible property only.
Add the SoC-specific one, to make it future proof.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>