ASoC: dt-bindings: sound: renesas,rz-ssi: Document DMA support

Document DMA support in binding document.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210719134040.7964-7-biju.das.jz@bp.renesas.com


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