nubus: Add expansion_type values for various Mac models

Add an expansion slot attribute to allow drivers to properly handle
cards like Comm Slot cards and PDS cards without declaration ROMs.
This clarifies the logic for the Centris 610 model which has no
Comm Slot but has an optional on-board SONIC device.

Cc: "David S. Miller" <davem@davemloft.net>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>