dts: bindings: Remove 'title:' and put all info. into 'description:'

Add any useful information from 'title:' to the 'description:' strings
(e.g. explanations of acronyms), and remove 'title:' as well as any
copy-pasted "this binding gives a ..." boilerplate.

Also clean some description strings up a bit.

Some other things could probably be cleaned up (replacing 'GPIO node'
with 'GPIO controller' on controllers for consistency, for example), but
I kept things close to the original to avoid accidentally messing up.

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