device.h: doc: Refactor to keep documentation infront of impl.

The ifdef'ing is re-ordering things so that the documentation is
either to far away from the implementation, or in the wrong
order (implementation and then documentation).

This commit reorders the macros and documentation so that we avoid
long-spanning #ifdef's and we keep the documentation at the head of
the implementation.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>