arm64: mm: Override arch_wants_pte_order()

Define an arch-specific override of arch_wants_pte_order() so that when
FLEXIBLE_THP is enabled, large folios will be allocated for anonymous
memory with an order that is compatible with arm64's contpte mappings.

arch_wants_pte_order() returns an order according to the following
policy: For the unhinted case, when THP is not requested for the vma,
don't allow anything bigger than 64K. This means we don't waste too much
memory. Additionally, for 4K pages this is the contpte size, and for
16K, this is (usually) the HPA size when the uarch feature is
implemented. For the hinted case, when THP is requested for the vma,
allow the contpte size for all page size configurations; 64K for 4K, 2M
for 16K and 2M for 64K.

Additionally, the THP and NOTHP order constants are defined using
Kconfig so it is possible to override them at build time.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>