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>