ArmPkg/PrePeiCore: use explicit UINTN cast in VOID* arithmetic

ArmPkg/PrePeiCore: use explicit UINTN cast in VOID* arithmetic

RVCT does not like the arithmetic involving a VOID* pointer that was
introduced in SVN r19163, so use explicit casts instead.

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@19172 6f19259b-4bc3-4df7-8a09-765794883524