perf: Fix idle notifier setup in arm64 PMU driver

The registration to idle notifier is missing in the PMU driver.
Registering it so that the idle events are properly handled, that
is, the counter values are updated before the cpu goes into idle.

Change-Id: If2a74ed8189f96fa28a05037d86e20e5b53b4a88
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>