dm: test: clk: Add test for ccf clk_set_rate()

Add a simple test case which sets clock rate to its current value.

Signed-off-by: Igor Prusov <ivprusov@salutedevices.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20231205232334.2931-3-ivprusov@salutedevices.com