mmc: dt: add DT binding for little-endian eSDHC host controller

Signed-off-by: Yangbo Lu <[e-mail]>
Signed-off-by: Ulf Hansson <[e-mail]>

Documentation/devicetree/bindings/mmc/fsl-esdhc.txt

···
   - voltage-ranges : two cells are required, first cell specifies minimum
     slot voltage (mV), second cell specifies maximum slot voltage (mV).
     Several ranges could be specified.
+  - little-endian : If the host controller is little-endian mode, specify
+    this property. The default endian mode is big-endian.
 
 Example: