PM / devfreq: memlat: optimize freq and stall_pct calculations

Currently, we calculate freq and stall_pct multiple times for
the same shared PMU counters. Calculate these once per cpu_grp
to prevent repeating the same calculation multiple times later.

Change-Id: I3b594e01ca50a583f45174e69a24f0b0eca32dde
Signed-off-by: Amir Vajid <avajid@codeaurora.org>
[avajid@codeaurora.org: made minor styling changes]
Signed-off-by: Amir Vajid <avajid@codeaurora.org>