drivers/perf: arm_pmu: avoid CPU device_node reference leak

of_cpu_device_node_get increments the reference count on the CPU
device_node, so we must take care to of_node_put once we've finished
with it.

This patch fixes the perf IRQ probing code to avoid the leak.

Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>