ARM: exynos: Fix a leaked reference by adding missing of_node_put

The call to of_get_next_child returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with warnings like:
    arch/arm/mach-exynos/firmware.c:201:2-8: ERROR: missing of_node_put;
        acquired a node pointer with refcount incremented on line 193,
        but without a corresponding object release within this function.

Cc: stable@vger.kernel.org
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>