MAINTAINERS: Add myself as the cpufreq maintainer

Since cpufreq has no official maintainer at the moment, I'm willing
to maintain it along some other power management core code I've been
maintaining already.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>