scripts/dts: Simplify get_all_compatibles()

 - Rename to all_compats(). The "compats" part is consistent with
   elsewhere.

 - Don't require an empty dictionary to be passed in. Build the
   result within the function instead.

 - Use more specific names (k, v -> child_name, child_node)

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>