pico-imx6ul: Add Falcon mode support

Falcon mode boots the kernel directly from SPL, without loading
the full U-Boot.

As pico-imx6ul does not have a GPIO for selecting Falcon versus normal
mode, enter in Falcon mode when the customer selects the
CONFIG_SPL_OS_BOOT option in menuconfig.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>