ARM: dts: imx: Use generic uart-has-rtscts DT property

As of commit 1006ed7e ("serial: imx: Use generic uart-has-rtscts
DT property"), the Freescale IMX UART driver recognizes the generic
"uart-has-rtscts" DT property, deprecating the vendor-specific
"fsl,uart-has-rtscts" DT property. Hence replace the latter by the
former in all DTS files.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>