pico-imx7d: Add instructions for booting in Falcon mode

Improve the README by adding instructions on how to boot using
Falcon mode, which allows the SPL to load the kernel directly,
without using U-Boot proper.

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