cpufreq: interactive: Add max_freq_hysteresis feature

Previously known as sampling down factor, max_freq_hysteresis
extends the period that interactive governor will stay at policy->max.
This feature is to accommodate short idle periods in an otherwise very
intensive workload.

When the feature is enabled, it ensures that once a CPU goes to max
frequency, it doesn't reduce the frequency for max_freq_hysteresis
microseconds from the time it first goes to idle.

Change-Id: Ia54985cb554f63f8c22d0b554a0a0f2ed2be038f
[junjiew@codeaurora.org: Resolved conflicts. Dropped changes to code
 that no longer exists. Trivial checkpatch fix. Renamed
 max_freq_idle_start_time to max_freq_hyst_start_time.]
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>