Document SoC specific bindings for R-Car M3-W+ (r8a77961) SoC.

Also as R8A7796 is now called R8A77960 so that update those
references.

Link: https://lore.kernel.org/r/20210409000020.2317696-1-yoshihiro.shimoda.uh@renesas.comSigned-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>