net: dsa: move dsa_tree_notify() and dsa_broadcast() to switch.c

There isn't an intuitive place for these 2 cross-chip notifier functions
according to the function-to-file classification based on names
(dsa_switch_*() goes to switch.c), but I consider these to be part of
the cross-chip notifier handling, therefore part of switch.c. Move them
there to reduce bloat in dsa2.c (the place where all code with no better
place to go goes).

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>