Introduce STMicroelectronics STM32MP1 platform

STM32MP1 is a microprocessor designed by STMicroelectronics,
based on a dual Arm Cortex-A7.
It is an Armv7-A platform, using dedicated code from TF-A.

STM32MP1 uses BL2 compiled with BL2_AT_EL3.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Mathieu Belou <mathieu.belou@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Pascal Paillet <p.paillet@st.com>