DT: Rename from dts.fixup to dts_fixup.h

The Zephyr configuration system uses many different files in many
different formats. It makes it a lot easier for users to understand
what these files do if when we use the correct file extensions.

To this end we rename the dts.fixup files to the correct file
extension '.h'.

This is a breaking change for out-of-tree fixup files. Such files will
be detected and given an appropriate error message.

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