dt-bindings: mmc: document alias support

As for I2C and SPI, it now is possible to reserve a fixed index for
mmc/mmcblk devices.
Signed-off-by: NMatthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://lore.kernel.org/r/20200901085004.2512-1-matthias.schiffer@ew.tq-group.comSigned-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc-controller.yaml

@@ -14,6 +14,10 @@ description: |
   that requires the respective functionality should implement them using
   these definitions.
 
+  It is possible to assign a fixed index mmcN to an MMC host controller
+  (and the corresponding mmcblkN devices) by defining an alias in the
+  /aliases device tree node.
+
 properties:
   $nodename:
     pattern: "^mmc(@.*)?$"