mmc: sdhci-of-arasan: Set controller to test mode when no CD bit

mmc: sdhci-of-arasan: Set controller to test mode when no CD bit

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.

When the device has the property xlnx,fails-without-test-cd the driver
changes the controller to test mode and sets test inserted to true to
make the controller function.

Signed-off-by: Zach Brown <zach.brown@ni.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>