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.

Also minor changes to increase code readability.

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