By using a common attr_groups array, the common arm_pmu code can set up
common files (e.g. cpumask) for us in subsequent patches.

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