dt-bindings: pinctrl: sh-pfc: Document r8a77961 support

Add DT binding documentation for the Pin Function 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: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191023122955.12420-2-geert+renesas@glider.be