arm64: dts: renesas: r8a774a1: Add audio support

Add sound support for the RZ/G2M SoC (a.k.a. R8A774A1).

This work is based on similar work done on the R8A7796 SoC
by Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>