ftrace: fix error with only one actor

When system has only one actor and call method trappy.summary_plots, it will report failure "zip argument #1 must support iteration".

This failure is cause by parsing allfreqs plots. Python always pass by value, so when there have only one actor the axis will pass by value and it's not a array type anymore. So finally zip will report it cannot support iteration. So in this case convert axis to array type.

Signed-off-by: Leo Yan leo.yan@linaro.org