mx6ul_14x14_evk: Fix SPL boot hang

mx6ul_14x14_evk: Fix SPL boot hang

Currently the following SPL hang is observed:

U-Boot SPL 2020.01-rc5-00079-g797eee36a1 (Jan 06 2020 - 11:24:09 -0300)
Trying to boot from MMC1
Card did not respond to voltage select!
spl: mmc init failed with error: -95
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

Fix it by moving the eSDHC2 initialization to SPL.

While at it, since this board uses DM_MMC all the esdhc board
code can be removed to make the code simpler.

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