dt-bindings: mmc: renesas,sdhi: Document RZ/V2N support

Add SDHI bindings for the Renesas RZ/V2N (a.k.a R9A09G056) SoC. Use
`renesas,sdhi-r9a09g057` as a fallback since the SD/MMC block on
RZ/V2N is identical to the one on RZ/V2H(P), allowing reuse of the
existing driver without modifications.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250326143945.82142-8-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>