arm: perf: factor out callchain code

The ARM callchain handling code is currently bundled with the ARM PMU
management code, despite the two having no dependency on each other.
This bundling has the unfortunate property of making callchain handling
depend on CONFIG_HW_PERF_EVENTS, even though the callchain handling
could be applied to software events in the absence of PMU hardware
support.

This patch separates the two, placing the callchain handling in
perf_callchain.c and making it depend on CONFIG_PERF_EVENTS rather than
CONFIG_HW_PERF_EVENTS, enabling callchain recording on kernels built
without hardware perf event support.

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