dts: clean up binding syntax checks

Commit 6bf761fc ("dts: Remove support
for deprecated DTS binding syntax") removed most of the support for
the 'legacy' bindings syntax.

A few straggler keys are still around in the bindings check code,
though. This allows some legacy keys which should cause errors to pass
silently instead.

Fix the error handling and print good errors for cases which are
removed, just in case someone is still using them somewhere.

Clean up some other error messages in the same function while we're
here.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>