sparc64: Add perf_event abstractions for orthogonal PMUs.

Starting with SPARC-T4 we have a seperate PCR control register
for each performance counter, and there are absolutely no
restrictions on what events can run on which counters.

Add flags that we can use to elide the conflict and dependency
logic used to handle older chips.

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