dt-bindings: serial: rs485: Add rs485-rts-active-high

Add rs485-rts-active-high property, this is a legacy property
used by 8250_omap.

This fixes the following make dt_binding_check warning:

Documentation/devicetree/bindings/serial/8250_omap.yaml:
rs485-rts-active-high: missing type definition
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/all/CAMuHMdUkPiA=o_QLyuwsTYW7y1ksCjHAqyNSHFx2QZ-dP-HGsQ@mail.gmail.com/
Fixes: 403e97d6 ("dt-bindings: serial: 8250_omap: add rs485-rts-active-high")
Cc: stable <stable@kernel.org>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231019154834.41721-1-francesco@dolcini.itSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/rs485.yaml

@@ -29,6 +29,10 @@ properties:
           default: 0
           maximum: 100
 
+  rs485-rts-active-high:
+    description: drive RTS high when sending (this is the default).
+    $ref: /schemas/types.yaml#/definitions/flag
+
   rs485-rts-active-low:
     description: drive RTS low when sending (default is high).
     $ref: /schemas/types.yaml#/definitions/flag