zynq: Enable CONFIG_DEFAULT_DEVICE_TREE

Enabled default dts files on respective pre-board config
files this is way MAKEALL will works. and it's upto user
to build specific dts by specifying at build time.

$ make zynq_zc70x_config
$ make -->  with default dts zynq-zc702.dts
or
$ make DEVICE_TREE=zynq-zc702 --> Same configuration with zynq-zc706.dts

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>