mmc: mmci: add qcom specific clk control

On Qcom SD card controller, cclk is mclk and mclk should be directly
controlled by the driver.
This patch adds support to control mclk directly in the driver, and also
adds explicit_mclk_control and cclk_is_mclk flags in variant structure giving
more flexibility to the driver.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>