On imx6qdl-sabresd the SDHC2 and SDHC3 are 8 bit-wide, so pass the bus-width
property to reflect that.

Otherwise the mmc driver will operate with the default bus-width value of 4.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>