dt-bindings: net: phy: add PoDL PSE property

Add property to reference node representing a PoDL Power Sourcing Equipment.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>