sched/cpufreq: sugov_cpu_is_busy for shared policy

Allow using sugov_cpu_is_busy() from sugov_update_shared(). This means
that the heuristic needs to return stable results across multiple calls
for a given CPU, even if there has been no utilization change since last
call.

sugov_cpu_is_busy() currently both checks business status and updates
the counters, so let's decouple the two actions.

Signed-off-by: Douglas RAILLARD <douglas.raillard@arm.com>