net: pse-pd: Add support for PSE PIs

The Power Sourcing Equipment Power Interface (PSE PI) plays a pivotal role
in the architecture of Power over Ethernet (PoE) systems. It is essentially
a blueprint that outlines how one or multiple power sources are connected
to the eight-pin modular jack, commonly known as the Ethernet RJ45 port.
This connection scheme is crucial for enabling the delivery of power
alongside data over Ethernet cables.

This patch adds support for getting the PSE controller node through PSE PI
device subnode.

This supports adds a way to get the PSE PI id from the pse_pi devicetree
subnode of a PSE controller node simply by reading the reg property.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://lore.kernel.org/r/20240417-feature_poe-v9-7-242293fd1900@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>