perf: arm_pmu: Move PMUv3-specific data

perf: arm_pmu: Move PMUv3-specific data

A few fields in struct arm_pmu are only used with PMUv3, and soon we
will need to add more for BRBE. Group the fields together so that we
have a logical place to add more data in future.

At the same time, remove the comment for reg_pmmir as it doesn't convey
anything useful.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Tested-by: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250218-arm-brbe-v19-v20-7-4e9922fc2e8e@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>