ArmPkg/Mmu: Fix bug of aligning new allocated page table

ArmPkg/Mmu: Fix bug of aligning new allocated page table

The code has a simple bug on calculating aligned page table address.
We can just use AllocateAlignedPages in MemoryAllocationLib instead.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18421 6f19259b-4bc3-4df7-8a09-765794883524