realtek: rtl93xx: dsa: Add support for port based mirroring

realtek: rtl93xx: dsa: Add support for port based mirroring

The RTL930X and RTL931X SoCs support port-based, flow-based, and
RSPAN-based mirroring. Like for other SoCs from the realtek target, only
the port based port mirroring can be exposed using Linux's tc subsystem.

The port_mirror_add() implementation was updated with the following
considerations for RTL93xx SoCs:

* mirrored packets must pass through the TX pipeline of the mirroring
  port, so they are subject to configuration such as VLAN tagging,
  remarking, and EVC
* when a packet hits both source ports (SPM) and destination port (DPM) of
  a mirror group, the egress port traffic will be mirrored

The port_mirror_del() function doesn't require any modifications.

Signed-off-by: Sharadanand Karanjkar <sk@simonwunderlich.de>
Co-developed-by: Sven Eckelmann <se@simonwunderlich.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>