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>

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -49,6 +49,9 @@ Optional properties:
 - mmc-hs400-enhanced-strobe: eMMC HS400 enhanced strobe mode is supported
 - dsr: Value the card's (optional) Driver Stage Register (DSR) should be
   programmed with. Valid range: [0 .. 0xffff].
+- no-sdio: controller is limited to send sdio cmd during initialization
+- no-sd: controller is limited to send sd cmd during initialization
+- no-mmc: controller is limited to send mmc cmd during initialization
 
 *NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
 polarity properties, we have to fix the meaning of the "normal" and "inverted"