feat(stm32mp1): select platform compilation either by flag or DT

To choose either STM32MP13 or STM32MP15, one of the two flags can be
set to 1 in the make command line. Or the platform selection can be
done with device tree name, if it begins with stm32mp13 or stm32mp15.

Change-Id: I72f42665c105b71a84b4952ef3fcd6c06ae4598c
Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>