sh: CPU hotplug support for SH-X3 SMP.

This wires up CPU hotplug for SH-X3 SMP CPUs. Presently only secondary
cores can be hotplugged given that the boot CPU has to contend with the
broadcast timer. When real local timers are implemented this restriction
can be lifted.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>