pm: use enum for device PM states

Move all PM_DEVICE_STATE_* definitions to an enum. The
PM_DEVICE_STATE_SET and PM_DEVICE_STATE_GET definitions have been kept
out of the enum since they do not represent any state. However, their
name has not been changed since they will be removed soon.

All drivers and tests have been adjusted accordingly.

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