dt-bindings: mmc: sprd: Add another optional clock documentation

For some Spreadtrum platforms like SC9860 platform, we should enable another
gate clock '2x_enable' to make the SD host controller work well. Thus add
documentation for this optional clock.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -14,6 +14,7 @@ Required properties:
 - clock-names: Should contain the following:
 	"sdio" - SDIO source clock (required)
 	"enable" - gate clock which used for enabling/disabling the device (required)
+	"2x_enable" - gate clock controlling the device for some special platforms (optional)
 
 Optional properties:
 - assigned-clocks: the same with "sdio" clock