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: b8a559feffb2 ("ARM: dts: imx: add Gateworks Ventana GW5600 support")
Signed-off-by: Fabio Estevam <[e-mail]>
Acked-by: Tim Harvey <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6qdl-gw560x.dtsi

···
 &uart1 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_uart1>;
-	uart-has-rtscts;
 	rts-gpios = <&gpio7 1 GPIO_ACTIVE_HIGH>;
 	status = "okay";
 };