ARM: dts: imx28-evk: remove duplicate property

Seems to be a left-over from an automatic merge.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>