dt-bindings: sound: renesas, rsnd: Document r8a774e1 bindings

Document SoC specific bindings for RZ/G2H (r8a774e1) SoC.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Link: https://lore.kernel.org/r/1594919915-5225-15-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>