sched/core: Move schedutil_cpu_util() to core.c

There is nothing schedutil specific in schedutil_cpu_util(), move it to
core.c and define it only for CONFIG_SMP.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lkml.kernel.org/r/c921a362c78e1324f8ebc5aaa12f53e309c5a8a2.1607400596.git.viresh.kumar@linaro.org