mx6cuboxi: Add support for eMMC booting

The HB2 boards as well as rev 1.5 soms support eMMC
booting as well as SDHC.  Add the infrastructure to support
booting these devices.

Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>