tools/power turbostat: Add a new GFXAMHz column that exposes gt_act_freq_mhz.

The column already present called GFXMHz reads from gt_cur_freq_mhz,
which represents the GT frequency that was requested, but power
management might not be able to do that. So the new column will display
what the actual frequency GT is running at.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>