arm64/mm: Unify CONT_PMD_SHIFT

Similar to how CONT_PTE_SHIFT is determined, this introduces a new
kernel option (CONFIG_CONT_PMD_SHIFT) to determine CONT_PMD_SHIFT.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200910095936.20307-3-gshan@redhat.com


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