The GFX controller is the internal graphics device used by the SoC
(opposed to the one connected via the PCIe device and used by the host).

This configures it with a framebuffer region and adds it to the command
line so kernel boot messages appear on the display.

Enabled for Romulus, Witherspoon, and the ASPEED AST2500 EVB.
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>