ARM: dts: zynq: correct and improve the model property of dt files

Replace the current value of the model property by a more accurate
description of each board (which includes the manufacturer), as some
of the boards had the same value ("Xilinx Zynq")

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>