doc: dts: add help for undefined reference to __device_dts_ord_N

This is an FAQ and we don't have a satisfactory page that describes
the current state of the world for how to troubleshoot. This leads
to frustrated users and duplicated support efforts. Try to improve
this by at least writing down what people have to do today.

If we ever improve the situation by adding a machine-readable map
between drivers, Kconfig options, and DT compatibles, we can hopefully
do better than this, but at least it's a start.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>