ARM: perf: move active_events into struct arm_pmu

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>