dm: spl: Support device tree when BSS is in a different section

At present in SPL we place the device tree immediately after BSS. This
avoids needing to copy it out of the way before BSS can be used. However on
some boards BSS is not placed with the image - e.g. it can be in RAM if
available.

Add an option to tell U-Boot that the device tree should be placed at the
end of the image binary (_image_binary_end) instead of at the end of BSS.

Note: A common reason to place BSS in RAM is to support the FAT filesystem.
We should update the code so that it does not use so much BSS.

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