pico-imx6ul: Add instructions for booting in Falcon mode

Falcon mode allows the SPL to load and jump directly to the kernel,
without loading U-Boot proper.

Add detailed step by step on how to use Falcon mode on pico-imx6ul.

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