arm64: perf: Refactor PMU init callbacks

The PMU init callbacks are already drowning in boilerplate, so before
doubling the number of supported PMU models, give it a sensible refactor
to significantly reduce the bloat, both in source and object code.
Although nobody uses non-default sysfs attributes today, there's minimal
impact to preserving the notion that maybe, some day, somebody might, so
we may as well keep up appearances.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>