Since commit commit 152038ea ("i.MX6UL: icore: Add SPL_OF_CONTROL
support") the OPOS6UL board doesn't boot anymore. Adding SPL_DM support
makes the board boot again.

Fixes: commit 152038ea ("i.MX6UL: icore: Add SPL_OF_CONTROL support")
Signed-off-by: NSébastien Szymanski <sebastien.szymanski@armadeus.com>