cpufreq: schedutil: clamp util to CPU maximum capacity

The code is to get the CPU util by accumulate different scheduling
classes and when the total util value is larger than CPU capacity
then it clamps util to CPU maximum capacity. So we can get correct util
value when use PELT signal but if with WALT signal it misses to clamp
util value.

On the other hand, WALT doesn't accumulate different class utilization
but it needs to applying boost margin for WALT signal the CPU util
value is possible to be larger than CPU capacity; so this patch is to
always clamp util to CPU maximum capacity.

Change-Id: I05481ddbf20246bb9be15b6bd21b6ec039015ea8
Signed-off-by: Leo Yan <leo.yan@linaro.org>