cgtqmx6eval: Add fastboot support

Tested basic fastboot commands, such as:

On the U-boot prompt:

=> fastboot 0

On the host PC:

$ fastboot getvar bootloader-version -i 0x0525
bootloader-version: U-Boot 2015.10-rc2-09654-g8f41d27
finished. total time: 0.000s

$ fastboot reboot -i 0x0525 --> board reboots fine.

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