dt-bindings: mmc: renesas,sdhi: Document RZ/G2L bindings

Document RZ/G2L SDHI controller bindings.

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/20210817090313.31858-3-biju.das.jz@bp.renesas.com


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