cpufreq: MAINTAINERS: Add git tree path for ARM specific updates

To reduce cpufreq maintenance load for Rafael, ARM specific patches
will be applied by me now.  Rafael will pull in these changes on a
regular basis (after rc's are released).  Additionally, I will be
sending pull request for every merge window and rc's (for fixes).

Web interface of my tree is available at:

https://git.linaro.org/gitweb?p=people/vireshk/linux.git;a=summary



Branch names will be: cpufreq-next and cpufreq-fixes.

This also prefixes the main tree path with "git ".

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