arm_pmu: acpi: factor out PMU<->CPU association

A subsequent patch will rework the ACPI probing of PMUs, and we'll need
to associate a CPU with a PMU in two separate paths.

Factor out the association logic into a helper function so that it can
be reused.

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

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Pierre Gondois <pierre.gondois@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-and-tested-by: Pierre Gondois <pierre.gondois@arm.com>
Link: https://lore.kernel.org/r/20220930111844.1522365-2-mark.rutland@arm.com


Signed-off-by: Will Deacon <will@kernel.org>