ARM: stm32: add initial support for STM32MP157

This patch adds initial support of STM32MP157 microprocessor (MPU)
based on Arm Cortex-A7. New Cortex-A infrastructure (gic, timer,...)
are selected if ARCH_MULTI_V7 is defined.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>