libs/utils/trace_analysis: warn on missing data
libs/utils/trace_analysis: warn on missing data

Let's generate at least a warning when no frequency events are present
in a trace. This usually means that we are using a fixed frequency governor
and the trace has been collected without fake cpu_frequency events injection
at the begin and end of the trace (e.g. using systrace/atrace)

Signed-off-by: Patrick Bellasi <431c86f368affbc3148c4669fbd465131f7e82cb@arm.com>

credp/lisa,ARM-software/lisa,credp/lisa,joelagnel/lisa,credp/lisa,credp/lisa,joelagnel/lisa,ARM-software/lisa,mdigiorgio/lisa,ARM-software/lisa,ARM-software/lisa,arnoldlu/lisa,bjackman/lisa,JaviMerino/lisa,mdigiorgio/lisa,arnoldlu/lisa,bjackman/lisa