xilinx: Fill git repository for Xilinx boards

All Xilinx SoCs have repository location filled already but boards are
covered by different fragment which is missing this link.
The patch is extending description with adding proper link to the same
repository.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>