ARM: shmobile: sh7372 A3SM CPUIdle support

Add CPUIdle support for the A3SM power domain on sh7372.

With this in place we can turn off more of the SoC during
run time and save power. Makes use of the recently introduced
sh7372 INTCA A3SM power domain wakeup support.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>