regulator: AXP20x: fix wrong call to of_find_node_by_name

The of_find_node_by_name function will search for a DT node named
"regulators" after the provided np node, but will not ensure that this node
is a child of np.
This might result in retrieving a "regulators" node that is not related to
the axp20x PMIC.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Mark Brown <broonie@linaro.org>