cpufreq: powernv: Select CPUFreq related Kconfig options for powernv

Enable CPUFreq for PowerNV. Select "performance", "powersave",
"userspace" and "ondemand" governors. Choose "ondemand" to be the
default governor.

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