pm: device: allow optional support of TURN_ON action

Some devices, e.g. SoC level devices like I2C peripheral, can never be
powerd off as they are always energized. Such devices can only go from an
active state or to a low power state (suspended). Allow them to simply
return -ENOTSUP when called with TURN_ON (or TURN_OFF).

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>