mmc: core: Add the vmmc/vmmcq regulator info

The core MMC code adds two (optional) regulator properites that drivers
should use to get their supplies. This is not documented anywhere so add
information on it.
Signed-off-by: NBen Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc.txt

@@ -75,6 +75,17 @@ Optional SDIO properties:
 - wakeup-source: Enables wake up of host system on SDIO IRQ assertion
 		 (Legacy property supported: "enable-sdio-wakeup")
 
+MMC power
+---------
+
+Controllers may implement power control from both the connected cards and
+the IO signaling (for example to change to high-speed 1.8V signalling). If
+the system supports this, then the following two properties should point
+to valid regulator nodes:
+
+- vqmmc-supply: supply node for IO line power
+- vmmc-supply: supply node for card's power
+
 
 MMC power sequences:
 --------------------