ARM: dts: imx28-duckbill: fix mmc settings

I2SE's duckbills are only equipped with a micro SD card slot and
thus only provide a 4-bit interface.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>