cpufreq: loongson2: use cpufreq_generic_init()

Use generic cpufreq_generic_init() routine instead of replicating the same code
here. This driver wasn't setting transition_latency and so is getting set to 0
by default. Lets mark it explicitly by calling the generic routine with
transition_latency as 0.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>