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>
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v2:
  - New.
1 file changed