Revert "ARM: mx6: add MMC2 boot device detection support in SPL"

Commit 54e4fcfa ("ARM: mx6: add MMC2 boot device detection
support in SPL") prevents UDOO neo board to boot:

Trying to boot from MMC2
port 1
MMC Device 1 not found
spl: could not find mmc device. error: -19
SPL: failed to boot from all boot devices

This reverts commit 54e4fcfa.

Signed-off-by: Breno Lima <breno.lima@nxp.com>