arm64/mm: Use CONT_PMD_SHIFT for ARM64_MEMSTART_SHIFT

ARM64_MEMSTART_SIZE needs to be aligned with CONT_PMD_SIZE on 16K page size
config. Hence just directly use CONT_PMD_SHIFT.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/1623663755-8949-1-git-send-email-anshuman.khandual@arm.com


Signed-off-by: Will Deacon <will@kernel.org>