ArmPlatformPkg/PrePeiCore: constify PPI globals

ArmPlatformPkg/PrePeiCore: constify PPI globals

Since PrePeiCore's .text section contains an AARCH64 exception
vector table, its 2 KB alignment propagates to other sections as
well. Since this is a SEC module, it should not have any writable
data in the first place, so change some non-const PPI globals to
const. The resulting binary has no .data section at all, which
saves 2 KB in the XIP image.

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@18188 6f19259b-4bc3-4df7-8a09-765794883524