ARM: dts: imx: Convert i.MX8M flash.bin image generation to binman

Rework the flash.bin image generation such that it uses the new binman
nxp_imx8mimage etype. This way, the flash.bin is assembled in correct
order using plain binman, without any workarounds or sections assembled
in special DT node order.

Reviewed-By: Tim Harvey <tharvey@gateworks.com>
Tested-By: Tim Harvey <tharvey@gateworks.com> # imx8mm_venice
Tested-by: Fabio Estevam <festevam@gmail.com> # imx8mm-evk and imx8mn-evk
Signed-off-by: Marek Vasut <marex@denx.de>