mm: Introduce arch_in_lazy_mmu_mode()

Introduce new arch_in_lazy_mmu_mode() API, which returns true if the
calling context is currently in lazy mmu mode or false otherwise. Each
arch that supports lazy mmu mode must provide an implementation of this
API.

The API will shortly be used to prevent accidental lazy mmu mode nesting
when performing an allocation, and will additionally be used to ensure
pte modification vs tlb flushing order does not get inadvertantly
swapped.

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