mx6cuboxi: Add emmc device tree suffix

Mainline and now the SolidRun 4.9 nxp based tree use the new
reorganization of device-tree files that separate out the emmc
into its own dtb.  u-boot will now look for -emmc in the device
tree name if one is detected.

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