perf: Add exclude_idle support for armv7 PMU

By adding the exclude_idle support, the driver would update
the PMU counter values into event->count right before it goes
into an idle state. This way, even if the client, from another
core, tries to read the value during this idle period, it would
return the last updated value before moving into idle without
waking the idle core up.

Change-Id: Iccc9a9d73e7079b27d0e2978bd286c4c464ce28f
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>