MAINTAINERS: add new entries for Armada 37xx cpufreq driver

This new driver belongs to the mvebu family, update the MAINTAINER file
to document it.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>