dt-bindings: leds: Document "netdev" trigger

Document the "netdev" trigger which is used to control LEDs by
network device activity. This is an existing trigger used in
existing DTs, document it so validation of those DTs would pass.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240708114653.18566-1-marex@denx.de


Signed-off-by: Lee Jones <lee@kernel.org>