arm64: dts: imx8mq-kontron-pitx-imx8m: 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: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-By: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>