drivers/rtc/rtc-88pm860x.c: use of_get_child_by_name()

Use of_get_child_by_name() to obtain reference to charger node instead of
of_find_node_by_name() which can walk outside of the parent node.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>