ArmPkg/ArmGic: Introduced ArmGicGetSupportedArchRevision()

ArmPkg/ArmGic: Introduced ArmGicGetSupportedArchRevision()

This function returns the revision of the GIC Architecture.
Some GICv3 controllers can work in GICv2 mode. Switching to an
older GIC revision is driven by the higher level exception level.
This function allows code to support any GIC revision at runtime.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16231 6f19259b-4bc3-4df7-8a09-765794883524