dt-bindings: mmc: broken clock stable indicator on arasan controllers

Some controllers immediately report that their internal clock is stable
after activating it even when the clock is not stable. When used in
conjunction with older/slower cards, this can result in:

    mmc0: error -84 whilst initialising SD card

This flag allows documenting and thus working around such a hardware
defect.
Signed-off-by: Helmut Grohne <h.grohne@intenta.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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

@@ -39,6 +39,8 @@ Optional Properties:
   - 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.
+  - xlnx,int-clock-stable-broken: when present, the controller always reports
+    that the internal clock is stable even when it is not.
 
 Example:
 	sdhci@e0100000 {