mmc: dw_mmc: Add the ability to set the ciu clock frequency

As of now we rely on code outside of the driver to set the ciu clock
frequency.  There's no reason to do that.  Add support for setting up
the clock in the driver during probe.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>