csky: Fixup perf probe failed

Current perf init will failed with:
[    1.452433] csky-pmu: probe of soc:pmu failed with error -16

This patch fix it up with adding CPUHP_AP_PERF_CSKY_ONLINE in
cpuhotplug.h.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>