mm: introduce arch_wants_lazy_mmu_mode()

powerpc decides at runtime whether the lazy MMU mode should be used.

To avoid the overhead associated with managing
task_struct::lazy_mmu_state if the mode isn't used, introduce
arch_wants_lazy_mmu_mode() and bail out of lazy_mmu_mode_* if it
returns false. Add a default definition returning true, and an
appropriate implementation for powerpc.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>