ARM: dts: imx28: Use generic uart-has-rtscts DT property

As of commit 182cdcb8 ("serial: mxs-auart: Use generic
uart-has-rtscts DT property"), the Freescale MXS AUART 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>