binman: imx8mimage: Generate FSPI header in binman instead of mkimage

binman: imx8mimage: Generate FSPI header in binman instead of mkimage

Stop depending on the current mkimage method of generating the FSPI
header, instead generate the FSPI header within binman itself. This
is more flexible, as the FSPI header properties can be configured
from within the board-specific DT instead of being hard-coded in
mkimage at build time.

Acked-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Marek Vasut <marex@nabladev.com>
Reviewed-by: Simon Glass <sjg@chromium.org>