cpufreq: intel_pstate: Add Icelake servers support in no-HWP mode

Users may disable HWP in firmware, in which case intel_pstate wouldn't load
unless the CPU model is explicitly supported.

Add ICELAKE_X to the list of CPUs that can register intel_pstate while not
advertising the HWP capability. Without this change, an ICELAKE_X in no-HWP
mode could only use the acpi_cpufreq frequency scaling driver.

See also commit d8de7a44

 ("cpufreq: intel_pstate: Add Skylake servers
support").

Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>