lib/igt_power: Add hwmon interface to igt_power

Modify igt_power to expose functions to read energy, power
using hwmon and rapl interface

Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>