arm: spl: Avoid setting up a duplicate global data structure

This is already set up in crt0.S. We don't need a new structure and don't
really want one in the 'data' section of the image, since it will be empty
and crt0.S's changes will be ignored.

As an interim measure, remove it only if CONFIG_DM is not defined. This
allows us to press ahead with driver model in SPL and allow the stragglers
to catch up.

Signed-off-by: Simon Glass <sjg@chromium.org>