ARM: shmobile: Update r8a7779 to check SCU for hotplug

Update the r8a7779 CPU Hotplug code to use SCU PSR
to wait for the target CPU core. Previously the
shared code in hotplug.c was used to let cpu_kill()
wait for cpu_die(). With this change in place the
r8a7779 SMP code does not depend on hotplug.c anymore.

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