net: pse-pd: Add support for setup_pi_matrix callback

Implement setup_pi_matrix callback to configure the PSE PI matrix. This
functionality is invoked before registering the PSE and following the core
parsing of the pse_pis devicetree subnode.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240417-feature_poe-v9-9-242293fd1900@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>