platform/x86/intel/uncore-freq: Display uncore current frequency

Add a new sysfs attribute "current_freq_khz" to display current uncore
frequency. This value is read from MSR 0x621.

Root user permission is required to read uncore current frequency.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20220204000306.2517447-4-srinivas.pandruvada@linux.intel.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>