arm64/mm: Ensure PGD_SIZE is aligned to 64 bytes when PA_BITS = 52

arm64/mm: Ensure PGD_SIZE is aligned to 64 bytes when PA_BITS = 52

Although the comment clearly states about PGD table's alignment requirement
(when PA_BITS = 52) but the subsequent BUILD_BUG_ON() tests size comparison
to 64 bytes instead. So change it as an actual alignment test.

Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>