dt-bindings: mmc: Add small binding note on level shifters

The VQMMC is often provided by a level shifter, so drop a small note
in the bindings that this can be the case and how that is done.
It is helpful information since this is pretty common.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220417144223.649201-1-linus.walleij@linaro.orgSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -298,7 +298,10 @@ properties:
 
   vqmmc-supply:
     description:
-      Supply for the bus IO line power
+      Supply for the bus IO line power, such as a level shifter.
+      If the level shifter is controlled by a GPIO line, this shall
+      be modeled as a "regulator-fixed" with a GPIO line for
+      switching the level shifter on/off.
 
   mmc-pwrseq:
     $ref: /schemas/types.yaml#/definitions/phandle