mmc: dt-bindings: Add optional cd-debounce-delay-ms

cd-gpios uses a fixed delay, 200ms, before detecting card after the card
is inserted. 200ms doesn't work for some platforms, so some host drivers
added their own properties for parsing that from DT, for instance,
dw_mmc and pxamci. That being said, it should also be tunable when using
cd-gpios.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>