arm64: dts: imx8m-venice: Remove incorrect 'uart-has-rtscts'

The following build warnings are seen when running:

make dtbs_check DT_SCHEMA_FILES=fsl-imx-uart.yaml

arch/arm64/boot/dts/freescale/imx8mm-venice-gw7903.dtb: serial@30860000: cts-gpios: False schema does not allow [[33, 3, 1]]
	From schema: Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml
arch/arm64/boot/dts/freescale/imx8mm-venice-gw7903.dtb: serial@30860000: rts-gpios: False schema does not allow [[33, 5, 1]]
	From schema: Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml
...

The imx8m Venice Gateworks boards do 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: 27c8f4cc ("arm64: dts: imx8mm-venice-gw72xx-0x: add dt overlays for serial modes")
Fixes: d9a9a7cf ("arm64: dts: imx8m{m,n}-venice-*: add missing uart-has-rtscts property to UARTs")
Fixes: 870f645b ("arm64: dts: imx8mp-venice-gw74xx: add WiFi/BT module support")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>