ARM: shmobile: Do not access sh7372 A4S domain internals directly

The sh7372_enter_suspend() routine checks the status field of the
generic PM domain object corresponding to the A4S domain in order to
check if it can turn that domain off when entering system sleep.
However, it shouldn't rely on the specific values of the generic
data structures this way, so make it use its own mechanism to
recognize when it is safe to turn that domain off.

For this purpos, introduce a boolean variable a4s_suspend_ready
that will be set by the A4S' suspend routine and unset by its
resume routine executed by rmobile_pd_power_down() and
__rmobile_pd_power_up(), respectively.

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