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>
diff --git a/fdts/stm32mp157c-ed1.dts b/fdts/stm32mp157c-ed1.dts
index 5d8817f..0fadffb 100644
--- a/fdts/stm32mp157c-ed1.dts
+++ b/fdts/stm32mp157c-ed1.dts
@@ -224,6 +224,7 @@
 	st,clksrc = <
 		CLK_MPU_PLL1P
 		CLK_AXI_PLL2P
+		CLK_MCU_PLL3P
 		CLK_PLL12_HSE
 		CLK_PLL3_HSE
 		CLK_PLL4_HSE
@@ -235,6 +236,7 @@
 	st,clkdiv = <
 		1 /*MPU*/
 		0 /*AXI*/
+		0 /*MCU*/
 		1 /*APB1*/
 		1 /*APB2*/
 		1 /*APB3*/