drivers/perf: arm_pmu: add common attr group fields

In preparation for adding common attribute groups, add an array of
attribute group pointers to arm_pmu, which will be used if the
backend hasn't already set pmu::attr_groups.

Subsequent patches will move backends over to using these, before adding
common fields.

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>

drivers/perf/arm_pmu.c

@@ -1037,6 +1037,9 @@ int arm_pmu_device_probe(struct platform_device *pdev,
 		goto out_free;
 	}
 
+	if (!pmu->pmu.attr_groups)
+		pmu->pmu.attr_groups = pmu->attr_groups;
+
 	ret = cpu_pmu_init(pmu);
 	if (ret)
 		goto out_free;