arm64: perf: Allow other entities to safely program the pmu

The pmu registers are now per cpu and protected using a spinlock.
If other entities need to program the pmu, they need to make sure
that they are not interfering with the perf driver's configuration.
Encapsulate the locking and unlocking functions and export them.

Change-Id: I42197b22f6be7b02b63e2faf76f2872f3c571973
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>