cpufreq: powernv: Adding fast_switch for schedutil

Adding fast_switch which does light weight operation to set the desired
pstate. Both global and local pstates are set to the same desired pstate.

Signed-off-by: Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>