Enable audio DMAC (= rcar-dmac) in r8a7793 device tree.

Based on similar work for the r8a7791 by Kuninori Morimoto.

Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

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v4
* Use GIC_SPI in interrupts properties