ARM: dts: imx6q-pistachio: Use 'uart-has-rtscts' property

The 'fsl,uart-has-rtscts' property is deprecated and it is recommended
to use the generic 'uart-has-rtscts' property instead.

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