board: add support for Xilinx zc706

This is a Xilinx Zynq (xc7z045) development board, based on Xilinx Zynq
(xc7z045).

This support is based on Avnet Zedboard defconfig/readme.

[Thomas:
 - minor fixes in the readme.txt
 - rename defconfig from zc706_defconfig to xilinx_zc706_defconfig.]

Signed-off-by: Jordi Montagne <jordi.montagne66@gmail.com>
Tested-by: Jean Burgat <jeanburgat33@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>