cpufreq: loongson2: Remove unused linux/sched.h headers

Since commit 759f534e(CPUFREQ: Loongson2: drop set_cpus_allowed_ptr()),
the header <linux/sched.h> is useless in oongson2_cpufreq.c, so remove it.

Signed-off-by: Hailong Liu <liu.hailong6@zte.com.cn>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>