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: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://lore.kernel.org/r/20200901085004.2512-1-matthias.schiffer@ew.tq-group.com


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