dt-bindings: net: pse-pd: Add bindings for PD692x0 PSE controller

Add the PD692x0 I2C Power Sourcing Equipment controller device tree
bindings documentation.

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-v9-11-242293fd1900@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>