net: dsa: add basic initial driver for MxL862xx switches

net: dsa: add basic initial driver for MxL862xx switches

Add very basic DSA driver for MaxLinear's MxL862xx switches.

In contrast to previous MaxLinear switches the MxL862xx has a built-in
processor that runs a sophisticated firmware based on Zephyr RTOS.
Interaction between the host and the switch hence is organized using a
software API of that firmware rather than accessing hardware registers
directly.

Add descriptions of the most basic firmware API calls to access the
built-in MDIO bus hosting the 2.5GE PHYs, basic port control as well as
setting up the CPU port.

Implement a very basic DSA driver using that API which is sufficient to
get packets flowing between the user ports and the CPU port.

The firmware offers all features one would expect from a modern switch
hardware, they are going to be added one by one in follow-up patch
series.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/ccde07e8cf33d8ae243000013b57cfaa2695e0a9.1770433307.git.daniel@makrotopia.org
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>