dt-bindings: mmc: renesas_sdhi: Document r8a77961 support

Document support for the SDHI controller in the Renesas R-Car M3-W+
(R8A77961) SoC.

Update all references to R-Car M3-W from "r8a7796" to "r8a77960", to
avoid confusion between R-Car M3-W (R8A77960) and M3-W+.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20191205134349.6410-2-geert+renesas@glider.be


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>