If the CPU assigned to the xgene PMU is taken offline, then subsequent
perf invocations on the PMU will fail:

  # echo 0 > /sys/devices/system/cpu/cpu0/online
  # perf stat -a -e l3c0/cycle-count/,l3c0/write/ sleep 1
    Error:
    The sys_perf_event_open() syscall returned with 19 (No such device) for event (l3c0/cycle-count/).
    /bin/dmesg may provide additional information.
    No CONFIG_PERF_EVENTS=y kernel support configured?

This patch implements a hotplug notifier in the xgene PMU driver so that
the PMU context is migrated to another online CPU should its assigned
CPU disappear.
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Hoan Tran <hoan.tran@amperecomputing.com>
[will: Made naming of new cpuhp_state enum entry consistent]
Signed-off-by: Will Deacon <will.deacon@arm.com>