arm64: defconfig: enable R8A77970 SoC

Enable the Renesas R-Car V3M (R8A77970) SoC in the ARM64 defconfig.

Suggested-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>