net: dsa: move notifier definitions to switch.h

Reduce bloat in dsa_priv.h by moving the cross-chip notifier data
structures to switch.h.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>