ArmVExpressPkg: avoid the use of ArmVExpressSysConfigLib at runtime

ArmVExpressPkg: avoid the use of ArmVExpressSysConfigLib at runtime

The ArmVExpressSysConfigLib library does not (and cannot) runtime
remap its MMIO registers, so use a NULL implementation of the
ArmPlatformSysConfigLib library class for DXE_RUNTIME_DRIVER
type modules.

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@17381 6f19259b-4bc3-4df7-8a09-765794883524