ARM: shmobile: Use shared SCU CPU Hotplug code on r8a7779

Update the r8a7779 specific CPU Hotplug code to make use of
the recently introduced shared SCU functions. The r8a7779
power domain hardware requires special power down handling
at ->kill() time.

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