sun4m: Add Sun CG3 framebuffer and corresponding OpenBIOS FCode ROM

sun4m: Add Sun CG3 framebuffer and corresponding OpenBIOS FCode ROM

The CG3 framebuffer is a simple 8-bit framebuffer for use with operating
systems such as early Solaris that do not have drivers for TCX.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
CC: Blue Swirl <blauwirbel@gmail.com>
CC: Anthony Liguori <aliguori@amazon.com>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Bob Breuer <breuerr@mc.net>
CC: Artyom Tarasenko <atar4qemu@gmail.com>