clk: s2mps11: Use of_get_child_by_name

of_find_node_by_name() walks over all nodes and can thus walk outside of
the parent node. Use of_get_child_by_name() instead.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>