arm: Use common outgoing-CPU-notification code

This commit removes the open-coded CPU-offline notification with new
common code.  In particular, this change avoids calling scheduler code
using RCU from an offline CPU that RCU is ignoring.  This is a minimal
change.  A more intrusive change might invoke the cpu_check_up_prepare()
and cpu_set_state_online() functions at CPU-online time, which would
allow onlining throw an error if the CPU did not go offline properly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>