mmc: sdhci: Factor out sdhci_enable_clk

Factor out sdhci_enable_clk from sdhci_set_clock
and make it EXPORT_SYMBOL so that it can be called.

Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>