cpufreq: speedstep-lib: Use monotonic clock

Wall time obtained from do_gettimeofday is susceptible to sudden jumps due to
user setting the time or due to NTP.

Monotonic time is constantly increasing time better suited for comparing two
timestamps.

Signed-off-by: Abhilash Jindal <klock.android@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>