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.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>