tools/power turbostat: extend --add option to accept /sys path

Previously, the --add option could specify only an MSR.

Here is is extended so an arbitrary /sys attribute,
as specified by an absolute file path name.

sudo ./turbostat --add /sys/devices/system/cpu/cpu0/cpuidle/state5/usage

Signed-off-by: Len Brown <len.brown@intel.com>