Thanks for your idea. I patched function is_cpu_model_supported for my cpu to return true. Syslog shows now the correct entry:
KERN: using Intel P-States: min 8, max 15, boost 23
KERN: found cpufreq module: power/cpufreq/intel_pstates/v1
KERN: using Intel C-States: C0, C1, C3, C4, C5, C6, C7
KERN: found cpuidle module: power/cpuidle/intel_cstates/v1
But all of my bench tests witch chess engines still give low values! 
For unknown reason p-state 23 (=2.3ghz) gets not activated by Haiku.
The cpu remains the whole time stuck at it’s default clock (p-state 15) with 1.5ghz.