clk: rk3399: Set empty for vopl assigned-clocks

During vidconsole probe, the device probe will try to
check whether the assigned clocks on that video console
node is initialized or not? and return an error if not.

But, unlike Linux U-Boot won't require to handle these
vopl assigned-clocks since core clocks are enough to
handle the video out to process.

So, mark them as empty in set_rate to satisfy clk_set_defaults
so-that probe happened properly.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>