stm32mp1: add example files for FIT generation

Add example of its files to generate FIT to start kernel
on ev1 or dk2 board with
- only kernel and dtb = fit_copro_kernel_dtb.its
- kernel, M4 copro firmware and dtb = it_copro_kernel_dtb.its

Add extlinux example to manage config in generated FIT.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>