mx6sabresd: Remove CONFIG_SPL_DM to decrease the SPL size

Currently the mx6qsabresd board does not boot:

U-Boot SPL 2019.07-rc2 (May 16 2019 - 14:28:55 -0300)
Trying to boot from MMC1
spl: could not find mmc device 0. error: -19
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

The reason for the boot failure is that that the SPL
size got greater than the 68KB limit (4KB header + 64KB max
size) as explained in include/configs/imx6_spl.h.

Remove the CONFIG_SPL_DM option, so that the SPL binary could
fit into the allowed size and the board can boot again.

Signed-off-by: Fabio Estevam <festevam@gmail.com>