bus: arm-ccn: fix hrtimer registration

The CCN PMU driver has a single hrtimer, used to simulate a periodic
interrupt on systems where the overflow interrupt is not possible to
use. The hrtimer is started when any event is started, and cancelled when
any event is stopped. Thus, stopping a single event is sufficient to
disable to hrtimer, and overflows (of other events) may be lost.

To avoid this, this patch reworks the hrtimer start/cancel to only occur
when the first event is added to a PMU, and the last event removed,
making use of the existing bitmap counting active events.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>