scripts/dts: Remove redundant and confusing dict.get() calls

dict.get(key) signals to people reading the code that you're not sure
whether the key exists. It returns None if it doesn't.

When the key is known to exist, dict[key] should be used. This also
helps catch bugs by raising an exception if the key is missing.

Similarly, whether a key in a dict should be tested with

    if key in dict:

instead of with

    if dict.get(key):

The second version signals that you both want to make sure that the
exists and that it's truthy (e.g., non-empty). That's confusing if a
simple existence check was meant.

There seems to be a bug in output_keyvalue_lines() where

    fd.write("%s=%s\n" % (entry, defs[node].get(a)))

can end up writing '...=None'. Removing the .get() makes it throw an
exception instead. Keep the .get() for now and don't attempt to fix the
bug.

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