pm: device: use z_device_get_all_static

Use the internal function z_device_get_all_static helper function
instead of using __device_start and __device_end directly. Some other
minor adjustments have been done (e.g. reduce *dev scope to the for
loop). An issue on the range of the for loop in _pm_devices has also
been fixed.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>