soc: qcom: smsm: fix child-node lookup

Fix child-node lookup during probe, which ended up searching the whole
device tree depth-first starting at the parent rather than just matching
on its children.

Note that the original premature free of the parent node has already
been fixed separately.

Also note that this pattern of looking up the first child node with a
given property is rare enough that a generic helper is probably not
warranted.

Fixes: c97c4090 ("soc: qcom: smsm: Add driver for Qualcomm SMSM")
Fixes: 3e8b5541 ("soc: qcom: smsm: fix of_node refcnting problem")
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>