ARM: dts: imx6qdl-gw5xxx: Remove the 'uart-has-rtscts' property

The 'uart-has-rtscts' property should be used when the board exposes the
native RTS and CTS UART pins.

On the imx6qdl-gw5xxx boards such pins are not used, so remove the
'uart-has-rtscts' property to make the hardware description correct.

Documentation/devicetree/bindings/serial/serial.txt states that
'uart-has-rtscts' and 'rts-gpios' properties are mutually exclusive.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>