vexpress64: Kconfig: move board definitions out of arch/arm

At the moment we define three "VExpress64" boards in arch/arm/Kconfig,
plus have a second Kconfig file in board/armltd/Kconfig.
One of those three boards is actually bogus (TARGET_VEXPRESS64_AEMV8A),
that stanza looks like being forgotten in a previous cleanup.

To remove the clutter from the generic Kconfig file, just define some
ARCH_VEXPRESS64 symbol there, enable some common options, and do the
board/model specific configuration in the board/armltd Kconfig file.

That allows to streamline and fine tune the configuration later, and
to also pull a lot of "non user choices" out of the defconfigs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>