Documentation: dt: kona-sdhci: Add clocks property

The Kona SDHCI block requires a clock that must be specified in the
device tree.  Update the documentation to reflect this requirement.
Signed-off-by: NTim Kryger <tim.kryger@linaro.org>
Reviewed-by: NMatt Porter <matt.porter@linaro.org>
Signed-off-by: NChristian Daudt <bcm@fixthebug.org>

Documentation/devicetree/bindings/mmc/kona-sdhci.txt

@@ -6,12 +6,16 @@ and the properties present in the bcm281xx SDHCI
 Required properties:
 - compatible : Should be "brcm,kona-sdhci"
 - DEPRECATED: compatible : Should be "bcm,kona-sdhci"
+- clocks: phandle + clock specifier pair of the external clock
+
+Refer to clocks/clock-bindings.txt for generic clock consumer properties.
 
 Example:
 
 sdio2: sdio@0x3f1a0000 {
 	compatible = "brcm,kona-sdhci";
 	reg = <0x3f1a0000 0x10000>;
+	clocks = <&sdio3_clk>;
 	interrupts = <0x0 74 0x4>;
 };