ARM: zynq: Introduce virtual defconfig

Add virtual defconfig for Zynq architecture using of_board dtb.
This defconfig should become generic config for all Zynq boards.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>