MAINTAINERS: cpufreq: add bmips-cpufreq.c

Add maintainer information for bmips-cpufreq.c.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>