soc: x86: apollo_lake: Turn .rst doc into .txt

In order to avoid a warning from Sphinx complaining that the
supported_features.rst file is not included in any ToC, rename it to
.txt so that Sphinx understands that this is only a snippet to be
included in other files.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>