ARM: SMP implementations are not supposed to return from smp_ops.cpu_die()

Although we allow recovery in this case, this is not supposed to be
the normal path for hotplugging a CPU back in.  This path only exists
to serve those rare platforms where it's not possible to power down
the CPU or reset the CPU.  This patch causes the kernel to print a
message when a platform uses this path.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>