boards: nrf: clean up DK board buttons in DTS

Our boards generally follow the following devicetree conventions:

- buttons are defined in the node with path /buttons
- alias sw0 is button 0, sw1 is button 1, and so on

Not all boards follow the convention, though. Align the oddballs for
consistency. This is useful for writing helper libraries that depend
on these conventions. Add a comment describing the odd situation for
nRF9160 DK while we are here.

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