arm: perf: unify perf_event{,_cpu}.c

Now that the arm_pmu framework is only used for CPU PMUs, there's no
reason to keep the pseudo-generic and CPU-specific framework portions
separate.

This patch folds the two into perf_event.c.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
[will: fixed up irq cfg to match upstream]
Signed-off-by: Will Deacon <will.deacon@arm.com>