powerpc/perf: Add missing of_node_put()s in imc-pmu.c

In update_events_in_group(), of_find_node_by_phandle() will return
a node pointer with refcount incremented. The reference should be
dropped with of_node_put() in the failure path or when it is not used
anymore.

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Link: https://lore.kernel.org/r/20220618071353.4059000-1-windhl@126.com