imx: mx6ul_14x14_evk: Fix SPL boot

When booting SPL on the board, met boot failure:
"
Trying to boot from MMC2
MMC Device 1 not found
spl: could not find mmc device 1. error: -19
SPL: failed to boot from all boot devices
"

Let's register the two mmc controllers in SPL stage to
avoid boot failure.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>