perf_counter, x86: introduce max_period variable

In x86 pmus the allowed counter period to programm differs. This
introduces a max_period value and allows the generic implementation
for all models to check the max period.

[ Impact: generalize code ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1241002046-8832-27-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>