This patch moves the active_events counter into struct arm_pmu, in
preparation for supporting multiple PMUs. This also moves
pmu_reserve_mutex, as it is used to guard accesses to active_events.
Signed-off-by: Mark Rutland <mark.rutland@arm.com> Reviewed-by: Will Deacon <will.deacon@arm.com> Reviewed-by: Jamie Iles <jamie@jamieiles.com> Signed-off-by: Will Deacon <will.deacon@arm.com>