realtek: dsa: convert trailer tag hack into separate module

realtek: dsa: convert trailer tag hack into separate module

DSA tagging currently works with a tuned trailer tagging. That means:

- realtek target uses tag_trailer for tagging
- there is a patch for the trailer tagger to write the target port not
  as a bitfield but as an integer

Make the tagging independent from upstream and hacky patches by providing
a new downstream driver. This can be aligned easier for future development.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21815
Signed-off-by: Robert Marko <robimarko@gmail.com>