arm: dts: zynq: Add digilent ZYBO board dts

It's a Zynq board similar in design to the currently supported ones.
512MB of RAM and UART1 is used.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>