docs: net: dsa: update information about multiple CPU ports

DSA now supports multiple CPU ports, explain the use cases that are
covered, the new UAPI, the permitted degrees of freedom, the driver API,
and remove some old "hanging fruits".

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>