net: ethtool: Add support for new power domains index description

Report the index of the newly introduced PSE power domain to the user,
enabling improved management of the power budget for PSE devices.

Signed-off-by: Kory Maincent (Dent Project) <kory.maincent@bootlin.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20250617-feature_poe_port_prio-v14-5-78a1a645e2ee@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>