mmc: omap_hsmmc: add clk_prepare and clk_unprepare

In preparation for OMAP moving to the Common Clock Framework (CCF)
add clk_prepare() and clk_unprepare() for the hsmmc clocks.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Acked-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>