vexpress64: kconfig: consolidate CONFIG_TARGET_VEXPRESS_AEMV8A_SEMI

We do not have to distinguish CONFIG_TARGET_VEXPRESS_AEMV8A_SEMI
from CONFIG_TARGET_VEXPRESS_AEMV8A.  Rename the former to the latter.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Steve Rae <srae@broadcom.com>
Cc: David Feng <fenghua@phytium.com.cn>