cpufreq: governor: Don't use 'timer' keyword

The earlier implementation of governors used background timers and so
functions, mutex, etc had 'timer' keyword in their names.

But that's not true anymore. Replace 'timer' with 'update', as those
functions, variables are based around updates to frequency.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>