mmc: sdhci-sprd: Add optional gate clock support

For the Spreadtrum SC9860 platform, we should enable another gate clock
'2x_enable' to make the SD host controller work well.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>