PM / devfreq: memlat: Look for min stall% in addition to ratio criteria

Some workloads doing memory access might appear memory latency bound even
though they might not actually be memory latency bound.

This error can happen when the core that's running the workload is very
parallelized or can do out of order executions, etc so not all memory
accesses would actually stall the core.

This can also happen when the the memory access monitoring capabilities
aren't ideal and end up counting more kinds of memory accesses than what
would be ideal. In this case, the IPM ratio can be lower than what it would
be if we had ideal monitoring capabilities.

To account for these errors, if the core has a stall cycle counting
capabilities, check for a minimum stall% before the workload is considered
memory latency bound. This would help reduce the inaccuracies, but is not a
replacement for IPM ratio scheme because the stall% method doesn't allow us
to detect which level of memory the workload is latency bound on, but the
IPM ratio does (based on which memory accesses we use for calculating the
ratio).

Change-Id: I4363d7848584e5562f6683b5ad6b0f99017ec71b
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>