add dc_set_clock
add dc_get_clock

this is for testing and diagnostics to get/set DPPCLK and DISPCLK.
Signed-off-by: NCharlene Liu <charlene.liu@amd.com>
Reviewed-by: NDmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: NLeo Li <sunpeng.li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>