ARM: PMU: re-export release_pmu symbol to modules

Commit b0e89590

 ("ARM: PMU: move CPU PMU platform device handling and
init into perf") inadvertently removed the EXPORT_SYMBOL_GPL on
release_pmu, so out-of-tree modules can no longer play nice with perf,
even if they tried in the first place.

This patch re-exports the symbol.

Reported-by: Jon Medhurst (Tixy) <jon.medhurst@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>