stm32mp: add binman support for STM32MP15x

Use binman to add the stm32image header on SPL binary for basic boot
or on U-Boot binary when it is required, i.e. for TF-A boot without FIP
support, when CONFIG_STM32MP15x_STM32IMAGE is activated.

The "binman" tool is the recommended tool for specific image generation.
This patch allows to suppress the config.mk file and it is a preliminary
step to manage FIT generation with binman.

The init_r parsing of U-Boot device tree to search the binman
information is not required for STM32MP15, so the binman library
can be removed in U-Boot (CONFIG_BINMAN_FDT is deactivated).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>