mmc: sh_mmcif: Document r8a7793 DT bindings

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
[geert: Rebased]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>