arm64: defconfig: ARCH_R8A7795: follow changed config symbol name

arm64: defconfig: ARCH_R8A7795: follow changed config symbol name

Completes commit fa9489c8922b ("soc: renesas: Add ARCH_R8A7795[01] for
existing R-Car H3") and commit 691b1272379f ("arm64: dts: renesas:
Remove use of ARCH_R8A7795").

CONFIG_ARCH_R8A7795 was split in CONFIG_ARCH_R8A77950 and
CONFIG_ARCH_R8A77951.

Link: https://lore.kernel.org/r/20200427134003.45188-4-max.krummenacher@toradex.com
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>