ArmVirtPkg: implement DT-based ArmGicArchLib

ArmVirtPkg: implement DT-based ArmGicArchLib

Since it is arguably incorrect to infer the GIC revision from CPU
ID and GIC feature registers on platforms that describe the GIC in
the device tree, this implements the library class ArmGicArchLib
tailored for such platforms.

The supported GIC revision is retrieved from the dynamic PCD that
is set based on the GIC DT node.

This means this library can only execute post DXE core, but this is
not a problem for any of the virt platforms.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Tested-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18102 6f19259b-4bc3-4df7-8a09-765794883524