ArmPlatformPkg/PrePeiCoreMPCore: use a unique GUID

ArmPlatformPkg/PrePeiCoreMPCore: use a unique GUID

PrePeiCoreMPCore reuses the GUID of its unicore sibling, which is
usually fine, given that platforms never include both. However, it
prevents us from creating a package .DSC that does include both, so
update the GUID to a fresh one.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>