PM / devfreq : Introduce a memory-latency governor

Use performance counters to detect the memory latency sensitivity
of CPU workloads and vote for higher DDR frequency if required.

Change-Id: Ie77a3523bc5713fc0315bd0abc3913f485a96e0e
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>
Suggested-by: Saravana Kannan <skannan@codeaurora.org>