ASoC: rsnd: Add r8a774c0 support

Document RZ/G2E (R8A774C0) SoC bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>