There's a cognitive load to figure out which mmc device node corresponds
to the eMMC flash, uSD card and WiFI SDIO module on the Snow, Peach Pi
and Pit boards.

So it's better to have comments in the DTS to make this more clear.
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
[krzk: Squashed three patches into one]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>