yaml: Remove redundant document separators

YAML document separators are needed e.g. when doing

  $ cat doc1.yaml doc2.yaml | <parser>

For the bindings, we never parse concatenated documents. Assume we don't
for any other .yaml files either.

Having document separators in e.g. base.yaml makes !include a bit
confusing, since the !included files are merged and not separate
documents (the merging is done in Python code though, so it makes no
difference for behavior).

The replacement was done with

    $ git ls-files '*.yaml' | \
        xargs sed -i -e '${/\s*\.\.\.\s*/d;}' -e 's/^\s*---\s*$//'

First pattern removes ... at the end of files, second pattern clears a
line with a lone --- on it.

Some redundant blank lines at the end of files were cleared with

    $ git ls-files '*.yaml' | xargs sed -i '${/^\s*$/d}'

This is more about making sure people can understand why every part of a
binding is there than about removing some text.

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