ArmVExpressPkg: use PSCI for system reset only on AARCH64 platforms

ArmVExpressPkg: use PSCI for system reset only on AARCH64 platforms

The PSCI specification covers both ARM and AARCH64, however, the
ARM Trusted Firmware (ATF) reference implementation is only available
for AARCH64, and PSCI firmware is not widely available for ARM platforms.

So use the EfiResetSystemLib implementation that uses PSCI calls only
on AARCH64, and revert to the Versatile Express-specific system register
interface (which is only available during boot time) on ARM platforms.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17927 6f19259b-4bc3-4df7-8a09-765794883524