cpufreq: MAINTAINERS: Add co-maintainer

Add Viresh as a co-maintainer of cpufreq framework.

This would mostly be for cpufreq core and ARM drivers but not
restricted to them.

Also add a pointer to the git tree cpufreq patches are pulled into.

[rjw: Changelog]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tentatively-acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>