App: Add no TZ context API support

Don't call tfm_nspm_register_client_id API if the NS OS doesn't support
TZ context management API.

Also update os_migration_guide_armv8m.md.

Change-Id: I44079f0414252ea4822f7623148ea351bb685d5f
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
4 files changed