ARM: perf: remove unused PMU probing code

The ARM perf backend can discover the type of PMU it needs to drive
either from DT or by probing a CPU it is running on. For
Cortex-A{5,7,15} there are no platforms in mainline not using dt, and
this probing won't work well for big.LITTLE systems with heterogeneous
PMUs.

This patch drops the probing for those CPUs, relying on information from
dt instead. Future platforms should describe their PMU(s) with dt.

Suggested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>