cpufreq: intel_pstate: fix struct cpudata::epp_cached kernel-doc

make C=1 currently gives the following warning:

drivers/cpufreq/intel_pstate.c:262: warning: Function parameter or struct member 'epp_cached' not described in 'cpudata'

Add the missing ":" to fix the trivial kernel-doc syntax error.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>