dt-bindings: mmc: renesas,sdhi: Document r8a7742 support

Document SDHI controller for RZ/G1H (R8A7742) SoC, which is compatible
with R-Car Gen2 SoC family.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marian-Cristian Rotariu <marian-cristian.rotariu.rb@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1589555337-5498-5-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com


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