MAINTAINERS: Add NuBus subsystem entry

This is legacy code but it might as well have an official maintainer.

Cc: linux-m68k@lists.linux-m68k.org
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>