Add an optional callback needed by some PMU features, e.g., AMD
BRS, to give a chance to the perf_events code to change its state before
a CPU goes to low power and after it comes back.

The callback is void when the PERF_NEEDS_LOPWR_CB flag is not set.
This flag must be set in arch specific perf_event.h header whenever needed.
When not set, there is no impact on the ACPI code.

Signed-off-by: Stephane Eranian <eranian@google.com>
[peterz: build fix]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220322221517.2510440-9-eranian@google.com