mx6sabresd: Add fastboot support

fastboot tool is a convenient way to flash the eMMC, so
add support for it.

Examples of usages:

On the mx6sabresd:

=> fastboot 0

On the Linux PC connected via USB:

$ sudo fastboot getvar bootloader-version -i 0x0525
bootloader-version: U-Boot 2018.01-00550-g7517cfe
finished. total time: 0.000s

$ sudo fastboot reboot -i 0x0525

(this causes the mx6sabresd to reboot)

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>