zynq: zybo: Move CONFIG_DEFAULT_DEVICE_TREE to defconfig

zynq: zybo: Move CONFIG_DEFAULT_DEVICE_TREE to defconfig

This is a MIME GnuPG-signed message.  If you see this text, it means that
your E-mail or Usenet software does not support MIME signed messages.
The Internet standard for MIME PGP messages, RFC 2015, was published in 1996.
To open this message correctly you will need to install E-mail or Usenet
software that supports modern Internet standards.

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>