ArmVExpressPkg: use named GUIDs for all VExpress variants

ArmVExpressPkg: use named GUIDs for all VExpress variants

Instead of using open coded GUIDs, define them only once, and refer
to them by name everywhere else.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17358 6f19259b-4bc3-4df7-8a09-765794883524