dt-bindings: mmc: sprd: Add another optional clock documentation

For some Spreadtrum platforms like SC9860 platform, we should enable another
gate clock '2x_enable' to make the SD host controller work well. Thus add
documentation for this optional clock.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>