x86/Documentation: Describe the Intel Hardware Feedback Interface

Start a documentation file to describe the purpose and operation of Intel's
Hardware Feedback Interface. Describe how this interface is used in Linux
to relay performance and energy efficiency updates to userspace.

Reviewed-by: Len Brown <len.brown@intel.com>
Suggested-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>