dt-bindings: mmc: renesas,sdhi: Add r8a774e1 support

Document SDHI controller for RZ/G2H (R8A774E1) SoC, which is compatible
with R-Car Gen3 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: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200809192001.19156-1-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>