perf counters: hw driver API

Impact: restructure code, introduce hw_ops driver abstraction

Introduce this abstraction to handle counter details:

 struct hw_perf_counter_ops {
	void (*hw_perf_counter_enable)	(struct perf_counter *counter);
	void (*hw_perf_counter_disable)	(struct perf_counter *counter);
	void (*hw_perf_counter_read)	(struct perf_counter *counter);
 };

This will be useful to support assymetric hw details, and it will also
be useful to implement "software counters". (Counters that count kernel
managed sw events such as pagefaults, context-switches, wall-clock time
or task-local time.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>