dt-bindings: net: pse-pd: Add another way of describing several PSE PIs

PSE PI setup may encompass multiple PSE controllers or auxiliary circuits
that collectively manage power delivery to one Ethernet port.
Such configurations might support a range of PoE standards and require
the capability to dynamically configure power delivery based on the
operational mode (e.g., PoE2 versus PoE4) or specific requirements of
connected devices. In these instances, a dedicated PSE PI node becomes
essential for accurately documenting the system architecture. This node
would serve to detail the interactions between different PSE controllers,
the support for various PoE modes, and any additional logic required to
coordinate power delivery across the network infrastructure.

The old usage of "#pse-cells" is unsuficient as it carries only the PSE PI
index information.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://lore.kernel.org/r/20240417-feature_poe-v9-8-242293fd1900@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>