ArmVirtualizationPkg: implement dummy RealTimeClockLib for Xen

ArmVirtualizationPkg: implement dummy RealTimeClockLib for Xen

This implements a dummy RealTimeClockLib for Xen, as there is no
guest interface to access the time kept by Xen that can be shared
between UEFI and the OS.

Contributed-under: TianoCore Contribution Agreement 1.0
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16977 6f19259b-4bc3-4df7-8a09-765794883524