dt-bindings: xilinx: zynq: Improve boards description

Change the description of some boards to make it similar
to the value of the model property from their respective
device-tree, using the format "<manufacturer> <model>"

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>