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>

Documentation/devicetree/bindings/serial/renesas,sci-serial.txt

@@ -76,6 +76,8 @@ Optional properties:
   - dmas: Must contain a list of two references to DMA specifiers, one for
 	  transmission, and one for reception.
   - dma-names: Must contain a list of two DMA names, "tx" and "rx".
+  - {cts,dsr,dcd,rng,rts,dtr}-gpios: Specify GPIOs for modem lines, cfr. the
+    generic serial DT bindings in serial.txt.
 
 Example:
 	aliases {