arm64: zynqmp: Introduce virtual defconfig

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

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