cpufreq: intel_pstate: Supply struct attribute description for get_aperf_mperf_shift()

Fixes the following W=1 kernel build warning(s):

 drivers/cpufreq/intel_pstate.c:293: warning: Function parameter or member 'get_aperf_mperf_shift' not described in 'pstate_funcs'

Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Remove line break ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>