ARM: mm: HugeTLB support for non-LPAE systems.

ARM: mm: HugeTLB support for non-LPAE systems.

Based on Bill Carson's HugeTLB patch, with the big difference being
in the way PTEs are passed back to the memory manager. Rather than
store a "Linux Huge PTE" separately; we make one up on the fly in
huge_ptep_get. Also rather than consider 16M supersections, we focus
solely on 2x1M sections.

To construct a huge PTE on the fly we need additional information
(such as the accessed flag and dirty bit) which we choose to store
in the domain bits of the short section descriptor. In order to use
these domain bits for storage, we need to make ourselves a client
for all 16 domains and this is done in head.S.

Storing extra information in the domain bits also makes it a lot
easier to implement Transparent Huge Pages, and some of the code in
pgtable-2level.h is arranged to facilitate THP support in a later
patch.

Non-LPAE HugeTLB pages are incompatible with the huge page migration
code (enabled when CONFIG_MEMORY_FAILURE is selected) as that code
dereferences PTEs directly, rather than calling huge_ptep_get and
set_huge_pte_at.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>