zynq: zybo: Move CONFIG_DEFAULT_DEVICE_TREE to defconfig

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Move CONFIG_DEFAULT_DEVICE_TREE from board config
directly to defconfig.

This patch removes compilation warning:
include/configs/zynq_zybo.h:24:0: warning: "CONFIG_DEFAULT_DEVICE_TREE"
redefined [enabled by default]
 #define CONFIG_DEFAULT_DEVICE_TREE zynq-zybo

Signed-off-by: Michal Simek <michal.simek@xilinx.com>