perf_counter: fix crash on perfmon v1 systems

Impact: fix boot crash on Intel Perfmon Version 1 systems

Intel Perfmon v1 does not support the global MSRs, nor does
it offer the generalized MSR ranges. So support v2 and later
CPUs only.

Also mark pmc_ops as read-mostly - to avoid false cacheline
sharing.

Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>