perf/arm-ni: Consolidate CPU affinity handling

perf/arm-ni: Consolidate CPU affinity handling

Since overflow interrupts from the individual PMUs are infrequent and
unlikely to coincide, and we make no attempt to balance them across
CPUs anyway, there's really not much point tracking a separate CPU
affinity per PMU. Move the CPU affinity and hotplug migration up to
the NI instance level.

Tested-by: Shouping Wang <allen.wang@hj-micro.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/00b622872006c2f0c89485e343b1cb8caaa79c47.1752256072.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>