ArmVExpressPkg: use ArmVExpressSysConfigRuntimeLib by default

ArmVExpressPkg: use ArmVExpressSysConfigRuntimeLib by default

Instead of using a NULL implementation of ArmPlatformSysConfigLib for
DXE_RUNTIME_DRIVER modules, which prevents them from accessing system
control registers even at boot time, use the new implementation that
only switches into a non-functional mode at runtime, and operates as
before otherwise.

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@17926 6f19259b-4bc3-4df7-8a09-765794883524