scripts/dts: Clean up Bindings.bindings()

Some more work will be done here later (yaml_list could be replaced by a
few plain variables, for example), but this is a start:

 - Extract the compat string from the 'compatible:' line directly in the
   regex, instead of separately

 - Flatten the code, turning

     if a:
       ...
       if b:
         ...
         if c:
           ...

   into

     if not a:
        continue

     ...

     if not b:
        continue

     ...

     if not c:
        continue

     ...

 - Add some comments

 - Use clearer naming

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