A memory mapped VGA controller for ZedBoard.

0. Pixel format
1. How to build the bitstream
2. Bare metal test
3. Work on Linux

0. Pixel format
 Pixels are stored in DDR memory starting on 0x01000000. Each 32-bit word
 stores 2 pixels. 
 ----------------------------------------------------------------------------
 | 31 - 28 | 27 - 24 | 23 - 20 | 19 - 16 | 15 - 12 | 11 - 8 | 7 - 4 | 3 - 0 |
 | UNUSED  | RED_1   | GREEN_1 | BLUE_1  | UNUSED  | RED_0 |GREEN_0 | BLUE_0|
 ----------------------------------------------------------------------------
 bits with XXX_0 contain 4-bit color value for pixel (N) and XXX_1 for (N + 1)

1. How to build the bitstream
 Create a new Vivado project for ZedBoard, add vga.vhd to the project
 sources and `Package IP' the current project.
 Create a new project and include in IP Repositories the path of last project.
 Create a block design with ZYNQ Processing System, Processor System Reset, an 
 AXI interconnect and the VGA block. Customize ZYNQ PS; add S_AXI_ACP port, a 
 PL Fabric Clock (FCLK_CLKx) of 102 MHz Actual Frequency, enable DDR and any 
 other blocks you need. Connect the master port of VGA block to interconnect
 and then to S_AXI_ACP of ZYNQPS. Make external VGA signals (r, g, b, hs, vs).
 Wire everything else and auto-asign addresses for vga mapping on Adress Editor.
 Add top_level_constrains.xdc file to constrains and generate the bitstream.

2. Bare metal test
 As bitstream is done, export hardware platform to SDK for C programming.
 Set a pointer to 0x1000000 and handle pixels on your own.
 Alternative - and faster - method is to program the bitstream to FPGA and 
 test it with xmd. Open xmd, program the fpga with the bitstream and test
 functionality with mwr commands.

3. Work on Linux
 After working test of bitstream bare-metal in order to make linux work with
 this hardware component you have to patch some files. Patches are on top of 
 linux kernel of Xilinx/xlnx 3.17 branch on github. Apply all patches from 
 linux_vga_patches directory from top linux kernel directory(e.g /usr/src/linux)
 Compile kernel, make uImage, a new dtb of arch/arm/boot/dts/zynq-zed.dts place
 it proper and reboot. 
 Note: simplefb is not the appropriete driver for this framebuffer because the
 r5g6b5 format is not what the pixel format is (for now).