ArmPlatformPkg/PrePi: Fixed register corruption

ArmPlatformPkg/PrePi: Fixed register corruption

The commit on Fri Dec 6 2013: "ArmPlatformPkg/ArmPlatformStackLib:
Do not directly use PcdArmPrimaryCore" has made the function
ArmPlatformStackSet() overwrites the registers r6 and r7.
These registers are used by PrePi to store the MPIDR and the the base
of the system memory region used by UEFI.

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@15167 6f19259b-4bc3-4df7-8a09-765794883524