dt-bindings: mmc: renesas,mmcif: Document r8a7742 DT bindings

Add support for r8a7742 SoC. Renesas RZ/G1H (R8A7742) MMCIF is identical
to the R-Car Gen2 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: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1588794695-27852-2-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>