Devices that do not require PM should just use NULL.
`device_pm_control_nop` is still kept as an alias to NULL untill all
in-tree usage is replaced with NULL.
Code relying on device_pm_control function now returns -ENOTSUP
(equivalent to calling device_pm_control_nop).
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
bfce935caf