arm64: defconfig: Replace ARCH_R8A7796 by ARCH_R8A77960

CONFIG_ARCH_R8A7796 was replaced by CONFIG_ARCH_R8A77960, cfr. commits
39e57e14 ("soc: renesas: Add ARCH_R8A77960 for existing R-Car
M3-W") and 24240845 ("soc: renesas: Remove ARCH_R8A7796").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200217101741.3758-1-geert+renesas@glider.be