ArmPkg/CpuDxe: Fix MMU initialization problem

During Mmu initialization in the CpuDxe, for a page table any bits set
in the 'NextSectionAttributes' are garbage and were set from bits that
are actually part of the pagetable address.  We clear it out to zero
so that the SyncCacheConfigPage will use the page attributes instead
of trying to convert the (bogus) section attributes into page
attributes.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Kurt Kennett <kurt.kennett@microsoft.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>