stm32mp1: add minimal support for co-processor Cortex-M4

STM32MP1 chip embeds a dual Cortex-A7 and a Cortex-M4.
The support for Cortex-M4 clocks is added when configuring the clock tree.
Some minimal security features to allow communications between A7 and M4
are also added.

Change-Id: I60417e244a476f60a2758f4969700b2684056665
Signed-off-by: Yann Gautier <yann.gautier@st.com>