net: dsa: realtek: rtl8365mb: add port_bridge_{join,leave}

Implement hardware offloading of bridge functionality. This is achieved
by using the per-port isolation registers, which contain a forwarding
port mask. The switch will refuse to forward packets ingressed on a
given port to a port which is not in its forwarding mask.

For each bridge that is offloaded, use the DSA-provided bridge number
for the Extended Filtering ID (EFID). When using Independent VLAN
Learning (IVL), the forwarding database is keyed with the tuple
{VID, MAC, EFID}. There are 8 EFIDs available (0~7), but we reserve the
default EFID 0 for standalone ports where learning is disabled. This
fits nicely because DSA indexes the bridge number starting from 1.

Because of the limited number of EFIDs, we have to set the
max_num_bridges property of our switch to 7: we can't offload more than
that or we will fail to offer IVL as at least two bridges would end up
having to share an EFID.

All ports start isolated, forwarding exclusively to CPU ports, and
with VLAN transparent, ignoring VLAN membership. Once a member in a
bridge, the port isolation is expanded to include the bridge members.
When that bridge enables VLAN filtering, the VLAN transparent feature is
disabled, letting the switch filter based on VLAN setup.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Co-developed-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Link: https://patch.msgid.link/20260606-realtek_forward-v13-8-b9e409687cbe@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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