ASoC: rsnd: Document r8a774b1 bindings

Document SoC specific bindings for RZ/G2N (r8a774b1) SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1570171940-42937-1-git-send-email-biju.das@bp.renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>