dt-bindings: net: dsa: add MaxLinear MxL862xx

Add documentation and an example for MaxLinear MxL86282 and MxL86252
switches.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/22a6a3c8c15b932ff4b7d0cd8863939f06a0c2b4.1770433307.git.daniel@makrotopia.org
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>