net: dsa: move headers exported by switch.c to switch.h

Reduce code bloat in dsa_priv.h by moving the prototypes exported by
switch.h into their own header file.

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>