device: remove inclusion of pm/device.h

The device PM subsystem _depends_ on device, not vice-versa. Devices
only hold a reference to struct pm_device now, and initialize this
reference with the value provided in Z_DEVICE_DEFINE. This requirement
can be solved with a forward struct declaration, meaning there is no
need to include device PM headers.

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