stm32mp1: Link BL2, BL32 and DTB in one binary

platform.mk is updated to have compilation rules for DTB, stm32image tool,
and the concatenation of the 3 binaries.
A new linker script and an assembly file are added to manage this.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Mathieu Belou <mathieu.belou@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>