arm64: defconfig: enable Renesas R8A7796 SoC

This patch enables the Renesas R8A7796 SoC in the arm64 defconfig.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>