arm64: introduce separated bits for mm_context_t flags

Currently mm->context.flags field uses thread_info flags which is not
the best idea for many reasons. For example, mm_context_t doesn't need
most of thread_info flags. And it would be difficult to add new mm-related
flag if needed because it may easily interfere with TIF ones.

To deal with it, the new MMCF_AARCH32 flag is introduced for
mm_context_t->flags, where MMCF prefix stands for mm_context_t flags.
Also, mm_context_t flag doesn't require atomicity and ordering of the
access, so using set/clear_bit() is replaced with simple masks.

Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>