PM / devfreq: memlat: Add writeback percentage as a lat condition

Certain L3 workloads are latency sensitive to cache snoop traffic and
this traffic is not directly quantified in the existing memlat scheme.

Use L2 writeback percentage as a metric to identify snoop traffic to
improve memlat.

Change-Id: I9d43375d96de5a199c6a87c55e5c1079549b23ce
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>