pm: device: move Z_DEVICE_PM_DEFINE_PM_SLOT to pm

The macro has been moved to the pm/device.h header, being now called by
the Z_PM_DEVICE_DEFINE macro. This means that a slot will only be
created if the device uses PM, thus reducing memory usage.

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