ARM: dts: imx6qdl-gw560x: Remove incorrect 'uart-has-rtscts'

The following build warning is seen when running:

make dtbs_check DT_SCHEMA_FILES=fsl-imx-uart.yaml

arch/arm/boot/dts/imx6dl-gw560x.dtb: serial@2020000: rts-gpios: False schema does not allow [[20, 1, 0]]
	From schema: Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml

The imx6qdl-gw560x board does not expose the UART RTS and CTS
as native UART pins, so 'uart-has-rtscts' should not be used.

Using 'uart-has-rtscts' with 'rts-gpios' is an invalid combination
detected by serial.yaml.

Fix the problem by removing the incorrect 'uart-has-rtscts' property.

Fixes: b8a559fe

 ("ARM: dts: imx: add Gateworks Ventana GW5600 support")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>