ARM: shmobile: Add shared SCU CPU Hotplug code

Add CPU Hotplug functions for SCU equipped mach-shmobile SoCs.

The functions shmobile_smp_scu_cpu_die() together with
shmobile_smp_scu_cpu_kill() perform basic shutdown and
allows checking of shutdown status. These are written
to work together with SMP boot code in  headsmp-scu.S.

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