boards: Rename .dts -> .dtsi where appropriate

DTS files which are only used as includes should have the proper suffix
according to spec. A .dts file is expected to contain a full devicetree
with a root node and a `/dts-v1/;` tag, at least after preprocessing.

For every board, ensure that the names of DTS includes end with ".dtsi",
and update the relevant references.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>