feat(stm32mp2): generate stm32 file

To be able to boot, STM32MPU platforms require the BL2 binary (together
with its DT) to be preceded with an STM32 header. Add the required
files and macro to properly generate this header.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I675de2c5cb733fe9d9e9baf76a941741a06dfac8
4 files changed