ARM: shmobile: Take cpuidle dependencies into account correctly

The sh7372's cpuidle code uses cpu_suspend() and cpu_resume() in
its implementation of the .enter() callback for deeper C-states, so
make ARCH_SH7372 select ARM_CPU_SUSPEND if CPU_IDLE is set to satisfy
all of the build dependencies (it has to be selected for CONFIG_PM
as well for the system suspend handling).

[rjw: Added the subject and changelog.]

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