ARM: shmobile: Add A4S cpuidle state on sh7372

Add a "C5" cpuidle state to the SH7372 SoC connected to the A4S power
domain in such a way that A4S may be turned off by cpuidle if all
I/O devices in that domain have been suspended (or do not have
attached drivers).

This requires some reorganization of the initialization of SH7372
power management which affects the the boards based on it, Mackerel
and AP4EVB.

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