serial: sh-sci: Add DT support for dedicated RTS/CTS

Add support for indicating the availability of dedicated lines for
RTS/CTS hardware flow control, using the standard "uart-has-rtscts" DT
property.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>