ARM: exynos: Fix refcount leak in exynos_map_pmu

In the Linux kernel, the following vulnerability has been resolved: ARM: exynos: Fix refcount leak in exynos_map_pmu of_find_matching_node() returns a node pointer with refcount incremented, we should use of_node_put() on it when not need anymore. Add missing of_node_put() to avoid refcount leak. of_node_put() checks null pointer.