device: treat a NULL device_pm_control as device_pm_control_nop

This is a step towards phasing out DEVICE_INIT and just having
DEVICE_DEFINE.  To make it a little easier on users or DEVICE_DEFINE
if they don't care about PM, they can just pass NULL for the
pm_control_fn paramater.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>