mmc: sdhci-sprd: Enable PHY DLL to make clock stable

For the Spreadtrum SD host controller, when we changed the clock to be
more than 52M, we should enable the PHY DLL which is used to track the
clock frequency to make the clock work more stable. Otherwise deviation
may occur of the higher clock.

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>