arm: zynq: Use separate device tree instead of embedded

Production boards should not use CONFIG_OF_EMBED. Fix this for the Zybo
boards.

The image to use now becomes u-boot-dtb.bin.

For example, the .bif file should contain a line like:

  [load = 0x04000000,startup=0x04000000]/path/to/u-boot-dtb.bin

instead of:

  [load = 0x04000000,startup=0x04000000]/path/to/u-boot.bin

When device tree is enabled we need to load u-boot-dtb.img. Change the
settings so that SPL does the right thing.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>