Blackfin: SMP: relocate blackfin_core_id() definition

Since we're breaking apart some inter-header dependencies to avoid more
circular loops, move the blackfin_core_id() definition to the func that
it is based upon.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>