ARM: dts: imx28-evk: add the RTS/CTS property for auart0

Add the RTS/CTS property for auart0 which means we enable the
DMA support for it.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>