power: max8925: 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>
Signed-off-by: Sebastian Reichel <sre@kernel.org>