cpupower: Add the function to get the sysfs value from specific table

Expose the helper into cpufreq header, then cpufreq driver can use this
function to get the sysfs value if it has any specific sysfs interfaces.

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>