perf_counter: powerpc: Change how processor-specific back-ends get selected

At present, the powerpc generic (processor-independent) perf_counter
code has list of processor back-end modules, and at initialization,
it looks at the PVR (processor version register) and has a switch
statement to select a suitable processor-specific back-end.

This is going to become inconvenient as we add more processor-specific
back-ends, so this inverts the order: now each back-end checks whether
it applies to the current processor, and registers itself if so.
Furthermore, instead of looking at the PVR, back-ends now check the
cur_cpu_spec->oprofile_cpu_type string and match on that.

Lastly, each back-end now specifies a name for itself so the core can
print a nice message when a back-end registers itself.

This doesn't provide any support for unregistering back-ends, but that
wouldn't be hard to do and would allow back-ends to be modules.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linuxppc-dev@ozlabs.org
Cc: benh@kernel.crashing.org
LKML-Reference: <19000.55529.762227.518531@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>