hw/m68k: add Nubus macfb video card

This patch adds support for a graphic framebuffer device.
This device can be added as a sysbus device or as a NuBus device.

It is accessed as a framebuffer but the color palette can be set.

Co-developed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20191026164546.30020-9-laurent@vivier.eu>