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

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -6,9 +6,6 @@ Interpreted by the OF core:
 - reg: Registers location and length.
 - interrupts: Interrupts used by the MMC controller.
 
-Required properties:
-- bus-width: Number of data lines, can be <1>, <4>, or <8>
-
 Card detection:
 If no property below is supplied, host native card detect is used.
 Only one of the properties in this section should be supplied:
@@ -17,6 +14,8 @@ Only one of the properties in this section should be supplied:
   - non-removable: non-removable slot (like eMMC); assume always present.
 
 Optional properties:
+- bus-width: Number of data lines, can be <1>, <4>, or <8>.  The default
+  will be <1> if the property is absent.
 - wp-gpios: Specify GPIOs for write protection, see gpio binding
 - cd-inverted: when present, polarity on the cd gpio line is inverted
 - wp-inverted: when present, polarity on the wp gpio line is inverted