dt-bindings: pse: tps23881: Clarify channels property description

Improve the channels property description to better explain the
relationship between physical delivery channels and PSE PI pairsets.
The previous description was unclear about how channels are referenced
and used in the port matrix mapping.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20250620-poe_doc_improve-v1-1-96357bb95d52@bootlin.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>