net: dsa: realtek: add LED drivers for rtl8366rb

This commit introduces LED drivers for rtl8366rb, enabling LEDs to be
described in the device tree using the same format as qca8k. Each port
can configure up to 4 LEDs.

If all LEDs in a group use the default state "keep", they will use the
default behavior after a reset. Changing the brightness of one LED,
either manually or by a trigger, will disable the default hardware
trigger and switch the entire LED group to manually controlled LEDs.
Once in this mode, there is no way to revert to hardware-controlled LEDs
(except by resetting the switch).

Software triggers function as expected with manually controlled LEDs.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>