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>