ARM: dts: imx6ul-kontron-n6x1x-s: Use the standard 'uart-has-rtscts'

The usage of the 'fsl,uart-has-rtscts' property is deprecated.

Use the standard 'uart-has-rtscts' instead.

Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>