PM / devfreq: Add timeout feature for cpufreq governor

Some devfreq devices might need their frequency to be set only for a short
time after the CPU frequency changes.

For such devices, add a "timeout" tunable that determines for how many
milliseconds the governor sets the device frequency based on the CPU
frequency. After "timeout" milliseconds from the CPU frequency change, the
governor will set the device frequency back to its minimum value.

Change-Id: I17fc972c0b03fab781864ce735013710c2df4647
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>