nubus: Generalize block resource handling

Scrap the specialized code to unpack video mode name resources and
driver resources. It isn't useful.
Instead, add a re-usable function to handle lists of block resources of
any kind, and descend into the video mode table resource directory.
Rename callers as nubus_get_foo(), consistent with their purpose and
with related functions in the same file.

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>