tifm: move common device management tasks from tifm_7xx1 to tifm_core

Some details of the device management (create, add, remove) are really
belong to the tifm_core, as they are not hardware specific.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>