Linux's "usbport" trigger is a bit specific one. It allows LED to follow
state of multiple USB ports which have to be selected additionally
(there isn't a single trigger for each port).

Default list of USB ports to monitor can be specified using
"trigger-sources" DT property. Theoretically it should be possible for
Linux to deduce applicable trigger based on the references nodes in the
"trigger-sources". It hasn't been implemented however (probably due to
laziness).

Milk spilled - we already have DT files specifying "usbport" manually -
allow that value in the binding. This fixes validation of in-kernel and
external DT files.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230316135546.9162-1-zajec5@gmail.com