drivers/perf: arm_pmu: implement CPU_PM notifier

When a CPU is suspended (either through suspend-to-RAM or CPUidle),
its PMU registers content can be lost, which means that counters
registers values that were initialized on power down entry have to be
reprogrammed on power-up to make sure the counters set-up is preserved
(ie on power-up registers take the reset values on Cold or Warm reset,
which can be architecturally UNKNOWN).

To guarantee seamless profiling conditions across a core power down
this patch adds a CPU PM notifier to ARM pmus, that upon CPU PM
entry/exit from low-power states saves/restores the pmu registers
set-up (by using the ARM perf API), so that the power-down/up cycle does
not affect the perf behaviour (apart from a black-out period between
power-up/down CPU PM notifications that is unavoidable).

Cc: Will Deacon <[email protected]>
Cc: Sudeep Holla <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Mark Rutland <[email protected]>
Acked-by: Ashwin Chaugule <[email protected]>
Acked-by: Kevin Hilman <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

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