kconfig: arm: move "armv8" define to arch/arm/Kconfig

Commit 2e07c249 (kconfig: arm: introduce symbol for ARM CPUs)
collected the default values of CONFIG_SYS_CPU into arch/arm/Kconfig.

This commit moves "armv8" to there for consistency.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Georges Savoundararadj <savoundg@gmail.com>