kconfig: armv8: move CONFIG_ARM64 to Kconfig

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/configs/vexpress_aemv8a_semi_defconfig b/configs/vexpress_aemv8a_semi_defconfig
index 24b868c..8fdf4e0 100644
--- a/configs/vexpress_aemv8a_semi_defconfig
+++ b/configs/vexpress_aemv8a_semi_defconfig
@@ -1,3 +1,3 @@
-CONFIG_SYS_EXTRA_OPTIONS="ARM64,SEMIHOSTING,BASE_FVP"
+CONFIG_SYS_EXTRA_OPTIONS="SEMIHOSTING,BASE_FVP"
 CONFIG_ARM=y
 CONFIG_TARGET_VEXPRESS_AEMV8A=y