ArmPlatformPkg/ArmPlatformLibNull: use declared PPI rather than module local var

ArmPlatformPkg/ArmPlatformLibNull: use declared PPI rather than module local var

We can refer to gArmMpCoreInfoPpiGuid directly if we declare it under
[Ppis] in the library's .inf so there is no need to copy it into a
module local variable.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19239 6f19259b-4bc3-4df7-8a09-765794883524