lisa.datautils: Add SignalDesc for clk_* events
lisa.datautils: Add SignalDesc for clk_* events

FIX

Add SignalDesc for clk_set_rate, clk_enable, clk_disable so that
requesting the dataframe for one of these will always provide the
occurence before the beginning of the trace window for each clock name.

credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa