powerpc/perf: Give generic PMU a nice name

When booting on a machine that uses the compat pmu driver we see this:

 [    0.071192] GENERIC_COMPAT performance monitor hardware support registered

Which is a bit shouty. Give it a nicer name.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220610044006.2095806-1-joel@jms.id.au