AT91: pm: use plain cpu_do_idle() for "wait for interrupt"

For power management at91_pm_enter() routine, use the cpu_do_idle() for a
rock solid "wait for interrupt" implementation.
For AT91SAM9 ARM 926 based chips, we can exceed the cache line length as
we can access RAM even while in self-refresh mode.
We keep plain access to CP15 for at91rm9200 as this feature is not
available: instructions have to be in a single cache line.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>