perf: Add support for exclude_idle attribute

Use the exclude_idle attribute of the perf events to avoid reading
PMUs of idle CPUs. The counter values are updated when CPU enters
idle and the saved value is returned when the idle CPU is queried
for that event provided the attribute is set in the perf_event.

Change-Id: I61f7a7474856abf67ac6dfd9e531702072e108a5
Signed-off-by: Patrick Fay <pfay@codeaurora.org>