ARM: at91: Consolidate time keeping and irq enable

Enable core cpuidle timekeeping and irq enabling and remove that
handling from this code.

Signed-off-by: Robert Lee <rob.lee@linaro.org>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Len Brown <len.brown@intel.com>