dt-bindings: leds: document new trigger-sources property

Some LEDs can be related to a specific device(s) described in the DT.
This property allows specifying such relations. E.g. USB LED should
usually be used to indicate some USB port(s) state.

Please note this binding is designed to be generic and not influenced by
any operating system design. Linux developers may find "trigger" part a
bit confusing since in Linux triggers are separated drivers. It
shouldn't define the binding though (we shouldn't add an extra level of
indirection).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>