ArmPlatformPkg/ArmVExpressPkg: add ArmPlatformSysConfigLib for runtime

ArmPlatformPkg/ArmVExpressPkg: add ArmPlatformSysConfigLib for runtime

This adds a ArmPlatformSysConfigLib implementation that is usable by
DXE_RUNTIME_DRIVER modules. Since the system registers that this library
encapsulates are not usable at runtime, this driver allows access to
those registers only at boot time.

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@17925 6f19259b-4bc3-4df7-8a09-765794883524