arm64: defconfig: Enable R8A77961 SoC

Enable the Renesas R-Car M3-W+ (R8A77961) SoC in the ARM64 defconfig.

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/20191023123342.13100-11-geert+renesas@glider.be