arm64: cpufeature: allow for version discrepancy in PMU implementations

Perf already supports multiple PMU instances for heterogeneous systems,
so there's no need to be strict in the cpufeature checking, particularly
as the PMU extension is optional in the architecture.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>