cpufreq: powernv: Use cpufreq_frequency_table.driver_data to store pstate ids

The .driver_data field in the cpufreq_frequency_table was supposed to
be private to the drivers. However at some later point, it was being
used to indicate if the particular frequency in the table is the
BOOST_FREQUENCY. After patches [1] and [2], the .driver_data is once
again private to the driver. Thus we can safely use
cpufreq_frequency_table.driver_data to store pstate_ids instead of
having to maintain a separate array powernv_pstate_ids[] for this
purpose.

[1]:
  Subject: cpufreq: don't print value of .driver_data from core
  From   : Viresh Kumar <viresh.kumar@ linaro.org>
  url    : http://marc.info/?l=linux-pm&m=139601421504709&w=2

[2]:
  Subject: cpufreq: create another field .flags in cpufreq_frequency_table
  From   : Viresh Kumar <viresh.kumar@linaro.org>
  url    : http://marc.info/?l=linux-pm&m=139601416804702&w=2



Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>