warp: Add USB Mass Storage support

With UMS support we are able to flash the eMMC from U-boot, which is very
convenient.

Add UMS support to make the eMMC flashing process easier.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>