mx6sabre_common: Add DFU support

Add DFU support.

Tested by flashing SPL and u-boot.img into SPI NOR flash with the
following commands:

=> setenv dfu_alt_info ${dfu_alt_info_spl}

=> run dfuspi

On the host PC:

$ sudo dfu-util -D SPL -a spl

On the target:

CTRL+C
=> setenv dfu_alt_info ${dfu_alt_info_img}

=> run dfuspi

On the host PC:

$ sudo dfu-util -D u-boot.img -a u-boot

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>