intel_pstate: add kernel parameter to force loading

To force loading on Oracle Sun X86 servers, provide one kernel command line
parameter

  intel_pstate = force

For those who are aware of the risk of no power capping capabily working
and try to get better performance with this driver.

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Tested-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: Linda Knippers <linda.knippers@hp.com>
Acked-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>