mmc: dt: bus-width can be an optional property

None of mmc drivers implements bus-width as a required device tree
property.  Instead, some drivers like atmel-mci, dw_mmc, sdhci-s3c
implement it as an optional one, and will force bus width to be 1
when the property is absent.  Let's change the common binding to
reflect what the drivers are usually doing.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>