latency_analysis: add statistics getter method

Right now, the plotLatency method returns a DF with statistics for the
latencies of a specified task. However, sometimes we could be
interested in getting these stats without necessarily plotting all the
data.

Let's move the statistics DF generation into an internal _get_latency_df
method, which can then be used by the plotLatency but also from an
additional DF gatter: _dfg_latency_stats_df.
Thus, this will make available a new:

   trace.data_frame.latency_stats_df()

which just return the overall latency stats for the specified task.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>