Documentation: mmc: add description for new no-sd* and no-mmc

This patch adds description for no-sd, no-sdio, no-mmc. We expose
these to DT as some of the controllers are unable to deal with
special cmd type due to hw limitation.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>