sparc64: Add a basic conflict engine in preparation for multi-counter support.

The hardware counter ->event_base state records and encoding of
the "struct perf_event_map" entry used for the event.

We use this to make sure that when we have more than 1 event,
both can be scheduled into the hardware at the same time.

As usual, structure of code is largely cribbed from powerpc.

Signed-off-by: David S. Miller <davem@davemloft.net>