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

Minimize the use of the bloated dsa_priv.h by moving the prototypes
exported by port.c to 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>