udoo_neo: Fixes for booting from the mSD card

This fixes booting from the mSD card from both SPL and when
using it for the OS booting. It also cleans up a few mmc
booting bits that are no longer needed.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>