device: improve device data initialization

The Z_DEVICE_STATE_DEFINE macro was conditioned by CONFIG_PM_DEVICE.
This is a problem if one day we have other conditional fields in the
device state field that need to be initialized. The approach has been
changed to have an always existing initializer for the PM field, that is
a no-op if device PM is not enabled.

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