regulator: tps6586x: get regulators node from parent node only

As per the devicetree binding document of TPS6586x, the "regulators"
subnode should be under the parent node, not outside of parent node.
Hence to get the regulator node, the correct call is
of_get_child_by_name() rather than of_find_node_by_name() which searches
the "regulators" node from the parent node to end of DTS file.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>