net: pse-pd: Fix missing PI of_node description

The PI of_node was not assigned in the regulator_config structure, leading
to failures in resolving the correct supply when different power supplies
are assigned to multiple PIs of a PSE controller. This fix ensures that the
of_node is properly set in the regulator_config, allowing accurate supply
resolution for each PI.

Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>