arm: perf: remove singleton PMU restriction

Now that we can describe PMUs in heterogeneous systems, the only item in
the way of perf support for big.LITTLE is the singleton cpu_pmu variable
used for OProfile compatibility.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>