cpufreq: Iterate over all the possible cpus to create powerstats.

For architectures which support a single policy for multiple cpus,
powerstats will not be initalized for all the cores. This change will
make sure powerstats is initialized for all the cores.

Bug: 21498425
Change-Id: I938f45e92ff6d5371c32c4d0e37274e6de66769c
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>