FROMLIST: arm64: reduce section size for sparsemem

Reducing the section size helps reduce wastage of reserved memory
for huge memory holes in sparsemem model. But having a much smaller
section size bits could break PMD mappings for vmemmap and wouldn't
accomodate the highest order page for certain page size granule configs.
It is determined that SECTION_SIZE_BITS of 27 (128MB) could be ideal
default value for 4K_PAGES that gives least section size without breaking
PMD based vmemmap mappings. For simplicity, 16K_PAGES could follow the
same as 4K_PAGES. And the least SECTION_SIZE_BITS for 64K_PAGES is 29
that could accomodate MAX_ORDER.

Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>

Link: https://lore.kernel.org/lkml/15cf9a2359197fee0168f820c5c904650d07939e.1610146597.git.sudaraja@codeaurora.org/


Bug: 175715646
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Iecc0369c595bcabe01182a85a5464c87df36c217