ARM: shmobile: Use shared SCU CPU Hotplug code on sh73a0

Update the sh73a0 specific CPU Hotplug code to make use of
the recently introduced shared SCU functions. The sh73a0
power control hardware relies on SCU_PM_POWEROFF with WFI
so the shared SCU code will as-is power down the hardware
as expected.

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