mmc: tmio-mmc: Add renesas, sdhi-r8a7791 to binding documentation

The driver already supports the r8a7791 SoC, and "renesas,sdhi-r8a7791"
is already in use.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Ian Molton <ian.molton@codethink.co.uk>
Cc: Chris Ball <chris@printf.net>
Cc: linux-mmc@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>