cpufreq: powernv-cpufreq: Fix a bunch of kerneldoc related issues

Repair problems with formatting and missing attributes/parameters, and
demote header comments which do not meet the required standards
applicable to kerneldoc.

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

 drivers/cpufreq/powernv-cpufreq.c:84: warning: Function parameter or member 'last_lpstate_idx' not described in 'global_pstate_info'
 drivers/cpufreq/powernv-cpufreq.c:84: warning: Function parameter or member 'last_gpstate_idx' not described in 'global_pstate_info'
 drivers/cpufreq/powernv-cpufreq.c:84: warning: Function parameter or member 'policy' not described in 'global_pstate_info'
 drivers/cpufreq/powernv-cpufreq.c:182: warning: Function parameter or member 'i' not described in 'idx_to_pstate'
 drivers/cpufreq/powernv-cpufreq.c:201: warning: Function parameter or member 'pstate' not described in 'pstate_to_idx'
 drivers/cpufreq/powernv-cpufreq.c:670: warning: Function parameter or member 't' not described in 'gpstate_timer_handler'
 drivers/cpufreq/powernv-cpufreq.c:670: warning: Excess function parameter 'data' description in 'gpstate_timer_handler'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>