blackfin: Use common outgoing-CPU-notification code

This commit removes the open-coded CPU-offline notification with new
common code.  This change avoids calling scheduler code using RCU from
an offline CPU that RCU is ignoring.  This commit is compatible with
the existing code in not checking for timeout during a prior offline
for a given CPU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Miao <realmz6@gmail.com>
Cc: <adi-buildroot-devel@lists.sourceforge.net>