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>
[avajid@codeaurora.org: updated attr definitions, removed exclude_idle flag and made minor styling changes]
Signed-off-by: Amir Vajid <avajid@codeaurora.org>