The `binding_filename` function, which resolves the binding
documentation file name from the binding yaml file name, made a very
crude assumption that the binding yaml file name will always have the
extension name of `yaml` -- this is no longer true after the commit
c453220006, which added support for both
`yaml` or `yml`.
This commit reworks the function such that its implementation is not
dependent on the extension name of the binding yaml file.
Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
99f1911520