soc: renesas: Add ARCH_R8A77961 for new R-Car M3-W+

Add CONFIG_ARCH_R8A77961 as a configuration symbol for the new Renesas
R-Car M3-W+ (R8A77961) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20191023123342.13100-4-geert+renesas@glider.be