ARM: shmobile: Introduce shmobile_smp_cpu_disable()

Introduce the shared CPU Hotplug function shmobile_smp_cpu_disable()
for mach-shmobile. It is useful for the case when all CPUs may be
hotplugged, including CPU 0.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>