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" <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Acked-by: Viresh Kumar <[e-mail]>
[ rjw: Remove line break ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/cpufreq/intel_pstate.c

···
  * @get_min:		Callback to get minimum P state
  * @get_turbo:		Callback to get turbo P state
  * @get_scaling:	Callback to get frequency scaling factor
+ * @get_aperf_mperf_shift: Callback to get the APERF vs MPERF frequency difference
  * @get_val:		Callback to convert P state to actual MSR write value
  * @get_vid:		Callback to get VID data for Atom platforms
  *