serial: sh-sci: Update DT binding documentation for dedicated RTS/CTS

serial: sh-sci: Update DT binding documentation for dedicated RTS/CTS

Some Renesas SCIF UARTs have dedicated lines for RTS/CTS hardware flow
control.  Whether these lines exist depends on SoC and UART instance
inside the SoC.  Whether these lines can be used for hardware flow
control depends on board wiring.

Amend the DT bindings with an optional property to indicate that RTS/CTS
hardware flow control lines exist, and can be used as such, according to
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>