mmc: mmci: Add required documentation for Device Tree bindings

This provides full documentation detailing each compulsory,
optional and device (variant) specific properties available
for use within the MMCI Device Tree.
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arndb@arndb.de>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/devicetree/bindings/mmc/mmci.txt

+* ARM PrimeCell MultiMedia Card Interface (MMCI) PL180/1
+
+The ARM PrimeCell MMCI PL180 and PL181 provides and interface for
+reading and writing to MultiMedia and SD cards alike.
+
+Required properties:
+- compatible             : contains "arm,pl18x", "arm,primecell".
+- reg                    : contains pl18x registers and length.
+- interrupts             : contains the device IRQ(s).
+- arm,primecell-periphid : contains the PrimeCell Peripheral ID.
+
+Optional properties:
+- wp-gpios               : contains any write protect (ro) gpios
+- cd-gpios               : contains any card detection gpios
+- cd-inverted            : indicates whether the cd gpio is inverted
+- max-frequency          : contains the maximum operating frequency
+- bus-width              : number of data lines, can be <1>, <4>, or <8>
+- mmc-cap-mmc-highspeed  : indicates whether MMC is high speed capable
+- mmc-cap-sd-highspeed   : indicates whether SD is high speed capable