m68k: Add serial controller to the NeXTcube machine

The NeXTcube uses a normal 8530 serial controller, so we can simply use
our normal "escc" device here.
While we're at it, also add a boot-serial-test for the next-cube machine,
now that the serial output works.

Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190831074519.32613-6-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>