We currently bundle the callchain handling code with the PMU code,
despite the fact the two are distinct, and the former can be useful even
in the absence of the latter.

Follow the example of arch/arm and factor the callchain handling into
its own file dependent on CONFIG_PERF_EVENTS rather than
CONFIG_HW_PERF_EVENTS.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>