realtek: dsa: Keep HW specific mirror code in SoC helper

realtek: dsa: Keep HW specific mirror code in SoC helper

Instead of using a lot of if-else blocks in the port mirror code, provide
SoC specific function which calculates the SoC specific portions. The
generic part of the port mirroring code can then simply operate on the
calculated register addresses and values.

Suggested-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Signed-off-by: Sven Eckelmann <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/20264
Signed-off-by: Robert Marko <robimarko@gmail.com>