ArmPlatformPkg/ArmVExpressPkg: Added support to differentiate ARMv8 FVP variants

There are three FVP variants for the Base and Foundation models:
- model with GICv2 legacy memory map (same location as the Versatile Express model)
- model with GICv2 and Base model memory map
- model with GICv3 and Base model memory map

The new code detects the variants to load the appropriate device tree.

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@16932 6f19259b-4bc3-4df7-8a09-765794883524