This commit adds VGA support for the Nexys A7/ Nexys 4 DDR.
The VGA is however limited to RGB443 instead of the full 12bit RGB444.
This is because IO D8 which is MSB for Blue, is also used for ETH int_n.
This makes the final output have a yellow tint.
We initially wanted to provide different level of support for the platforms/targets, mainly
to avoid too much maintenance and let each contributor update its contributed platforms and
targets, but it's easier to update all platforms/targets all-together when LiteX evolves or
changes (and that's what has been done on litex-boards since the creation of the repository).
So let just simplify things and avoid this differentiation.
Florent Kermarrec
Sahaj Sarup