ARM i.MX dtsi: Add default bus-width property for esdhc controller

According to Documentation/devicetree/bindings/mmc/mmc.txt bus-width
is a mandatory property. While this is currently enforced nowhere, it's
a good habit to just add the property now to allow to add common helper
functionality for the mmc property parsing later.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>