kconfig: zynq: Add ZYBO board

Add a defconfig and Kconfigury for the Digilent ZYBO board.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>