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>

Documentation/arm/stm32/stm32mp157-overview.rst

@@ -0,0 +1,19 @@
+STM32MP157 Overview
+===================
+
+Introduction
+------------
+
+The STM32MP157 is a Cortex-A MPU aimed at various applications.
+It features:
+
+- Dual core Cortex-A7 application core
+- 2D/3D image composition with GPU
+- Standard memories interface support
+- Standard connectivity, widely inherited from the STM32 MCU family
+- Comprehensive security support
+
+:Authors:
+
+- Ludovic Barre <ludovic.barre@st.com>
+- Gerald Baeza <gerald.baeza@st.com>