Initial gateware code for VGA capture

[rohitk-singh]

Changes:

• Added gateware code for vga capture in gateware/vga
• Added atlys_vga.py target for vga capture
• Modified atlys platform file with vga pins
• Added some firmware code for vga capure in ad9984a.c and vga_in.c

Doesn't have to merged compulsorily. If any suggestions or refactoring/restructuring ideas are there, they are welcome. Any comments also welcome.

Have to add rest of the firmware code for capture to work. There are some conflicts with the code, so haven't committed them yet. Looking into them right now

[mithro]

Hi @rohit91,

Generally things are looking pretty good but there are some small formatting issues. Can you please do the following things;

• Run the python code through pep8. Due to the way migen code is generally formatted, you won't be able to be 100% pep8 clean, but it should at least show you where you have some formatting problems.
• Fix tabs/spaces in the C code. It seems most of the existing code uses tabs.

[mithro]

@enjoy-digital Can you take a close look at the migen code?

[mithro]

@rohit91 Was this the code that @CarlFK got working today?

[rohitk-singh]

Starting afresh.

Changes:

• Initial gateware code for vga capture in gateware/vga
• Modified platforms/atlys.py platform file by adding vga signals

@mithro Can you look at this and see if it can be merged or not. pep8 checks are ok (only visual indentation warnings and few >79 line-length warnings). I have incorporated all of @enjoy-digital's suggestions he had after his review. This gateware code is all that is necessary for vga capture. I'll be creating separate pull requests each for firmware and for target.