DT: mmc: sh_mmcif: document R8A7794 support

Renesas R8A7794 SoC also has the MMCIF controller.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>