cpufreq: Loongson1: Add cpufreq driver for Loongson1B

This patch adds cpufreq driver for Loongson1B which
is capable of changing the CPU frequency dynamically.

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>