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>

Documentation/devicetree/bindings/mmc/tmio_mmc.txt

@@ -18,6 +18,7 @@ Required properties:
 		"renesas,sdhi-r8a7778" - SDHI IP on R8A7778 SoC
 		"renesas,sdhi-r8a7779" - SDHI IP on R8A7779 SoC
 		"renesas,sdhi-r8a7790" - SDHI IP on R8A7790 SoC
+		"renesas,sdhi-r8a7791" - SDHI IP on R8A7791 SoC
 
 Optional properties:
 - toshiba,mmc-wrprotect-disable: write-protect detection is unavailable