ARM: shmobile: Add CPU notifier based SCU boot vector code

Add CPU notifiers for the shared mach-shmobile SCU code
to allow removal of the shared SCU boot_secondary code.

Regarding notifiers, at CPU_UP_PREPARE time the SMP boot
vector is initialized so secondary CPU cores can boot.

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