imx8mm_venice: switch to use binman to pack images

Use binman to pack images.

Note that imx8mm_venice supports several boards via multiple DTB's thus
in the fit node we must use:
- fit,fdt-list = "of-list"
- fdt-SEQ
- config-SEQ

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>