intel_pstate: Knights Landing support

 1. Add Knights Landing (KNL) CPUID to the list of CPUIDs supported by
    the intel_pstate driver.

 2. Add a new cpu_default structure for KNL since KNL has a slightly
    different mechanism to get turbo pstates from MSRs.

Signed-off-by: Dasaratharaman Chandramouli <dasaratharaman.chandramouli@intel.com>
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
[ rjw: Subject and changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>