ARM: shmobile: Introduce shared SCU SMP boot code

Add SMP boot functions for SCU equipped mach-shmobile SoCs.

At this point shmobile_smp_scu_prepare_cpus() controls the SCU and
installs boot fn and arg, while shmobile_smp_scu_boot_secondary()
currently does nothing. In the future the boot function and arg
install code will be reworked, so the empty function is ground
work for that.

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