mx6cuboxi: enable OF_CONTROL and DM in SPL

In order to take the beneficts of DT and DM in SPL, like reusing the code
and avoid redundancy, enable SPL_OF_CONTROL, SPL_DM and SPL_DM_MMC.

With this new configuration SPL image is 50 KB, higher than the
38 KB from the previous version, but it still under the 68 KB limit.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>