ARM: dts: imx6sl-warp: Pass 'bus-width' property

USDHC2 port uses all the 8 data signals, so pass the 'bus-width' property
accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>