serial: sh-sci: Update DT binding documentation for GPIO modem lines

Amend the DT bindings for the Renesas SCI driver to allow describing
optional GPIO-controlled modem lines, which can be used where dedicated
modem lines are not available.

The property naming is dictated by the Generic Serial DT Bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>