Different drivers present the current cpu core frequency in different
sysfs files. Some present it in cpuinfo_cur_freq, some in scaling_cur_freq,
and some actually present it in both.
This patch attempts to open one, if that fails, tries the other.
Fixes: d550a8cc31 ("app/test: enhance power manager unit tests")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>