m68k: Add NeXTcube framebuffer device emulation

m68k: Add NeXTcube framebuffer device emulation

The NeXTcube uses a linear framebuffer with 4 greyscale colors and
a fixed resolution of 1120 * 832.
This code has been taken from Bryce Lanham's GSoC 2011 NeXT branch at

 https://github.com/blanham/qemu-NeXT/blob/next-cube/hw/next-fb.c

and altered to fit the latest interface of the current QEMU (e.g.
the device has been "qdev"-ified etc.).

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190831074519.32613-2-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>