ARM: stm32: add support for STM32MP151

This patch adds initial support of STM32MP151 microprocessor (MPU)
based on Arm Cortex-A7. New Cortex-A infrastructure (gic, timer,...)
are selected if ARCH_MULTI_V7 is defined.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Roan van Dijk <roan@protonic.nl>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>