ARM: imx: cpuidle: use init/exit common routine

The code intializes the cpuidle driver at different places.
The cpuidle driver for :
  * imx5 : is in the pm-imx5.c, the init function is in cpuidle.c
  * imx6 : is in cpuidle-imx6q.c, the init function is in cpuidle.c
           and cpuidle-imx6q.c

Instead of having the cpuidle code spread across different files,
let's create a driver for each SoC and use the common register function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>