dt: sdhci-of-arasan: Add device tree option xlnx, fails-without-test-cd

The sdhci controller on xilinx zynq devices will not function unless
the CD bit is provided. http://www.xilinx.com/support/answers/61064.html
In cases where it is impossible to provide the CD bit in hardware,
setting the controller to test mode and then setting inserted to true
will get the controller to function without the CD bit.

The device property "xlnx,fails-without-test-cd" will let the arasan
driver know the controller does not have the CD line wired and that the
controller does not function without it.
Signed-off-by: Zach Brown <zach.brown@ni.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/arasan,sdhci.txt

@@ -36,6 +36,9 @@ Optional Properties:
   - #clock-cells: If specified this should be the value <0>.  With this property
     in place we will export a clock representing the Card Clock.  This clock
     is expected to be consumed by our PHY.  You must also specify
+  - xlnx,fails-without-test-cd: when present, the controller doesn't work when
+    the CD line is not connected properly, and the line is not connected
+    properly. Test mode can be used to force the controller to function.
 
 Example:
 	sdhci@e0100000 {