perf/powerpc: reset event hw state when adding it to the PMU

When adding an event to the PMU with PERF_EF_START the STOPPED and UPTODATE
flags need to be cleared in the hw.event status variable because they are
preventing the update of the event count on overflow interrupt.

Signed-off-by: Alexandru-Cezar Sardan <alexandru.sardan@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>