Introduce STM32MP15 function and defines to prepare the STM32MP13 introduction. Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com> Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com> Change-Id: I909b205e73dcf207e0216aae5905c3c52472020e