mx6sabreauto: Remove CONFIG_SPL_DM to decrease the SPL size

The i.mx6 SPL binary cannot be bigger then 68K, while with the current
defconfig for sabreauto it's only about 56K as soon as USB support gets
added the size will overflows.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Fabio Estevam <festevam@gmail.com>