scripts: edtlib.py: Deprecate 'title:'

Most bindings look something like this:

    title: Foo

    description: This binding provides a base representation of Foo

That kind of description doesn't add any useful information, as it's
just the title along with some copy-pasted text. I'm not sure what "base
representation" was supposed to mean originally either.

Many bindings also put something that's closer to a description in the
title, because it's not clear what's expected or how the title is used.
In reality, the title isn't used anywhere. 'description:' on the other
hand shows up as a comment in the generated header.

Deprecate 'title:' and generate a long informative warning if it shows
up in a binding.

Next commits will clean up the 'description:' strings (bringing them
closer to 'title:' in most cases) and remove 'title:' from all bindings.

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