PM / devfreq: Generic cpufreq governor

This devfreq governor is a generic implementation that can scale any
devfreq device based on the current CPU frequency of all ONLINE CPUs. It
allows for specifying CPU freq to devfreq mapping for specific devices.
When such a mapping is not present, it defaults to scaling the device
frequency in proportion to the CPU frequency.

Change-Id: I8d3b1fea26572defbad2ce8b6cf87930caa25ca6
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>