mmc: davinci: prepare clock

When trying to use this driver with the common clock framework, enabling
the clock fails because it was not prepared. This fixes the problem by
calling clk_prepare and clk_enable in a single function. Ditto for
clk_disable_unprepare.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>