pico-imx7d: Add fastboot support

pico-imx7d: Add fastboot support

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

Examples of usages:

On the pico-imx7d U-Boot prompt:

=> fastboot 0

On the Linux PC connected via USB:

1. Retrieving the U-Boot version

$ sudo fastboot getvar bootloader-version -i 0x0525
bootloader-version: U-Boot 2018.07-rc1-03888-gde846f9
finished. total time: 0.000s

2. Resetting the board

$ sudo fastboot reboot -i 0x0525

(this causes the pico-imx7d to reboot)

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>