stm32mp1: add trusted boot with TF-A

Add support of trusted boot, using TF-A as first stage bootloader,
The boot sequence is
  BootRom >=> TF-A.stm32 (clock & DDR) >=> U-Boot.stm32

The TF-A monitor provides secure monitor with support of SMC
- proprietary to manage secure devices (BSEC for example)
- PSCI for power

The same device tree is used for STMicroelectronics boards with
basic boot and with trusted boot.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 4a23e32..7645470 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1406,8 +1406,10 @@
 	help
 	  Support for STM32MP SoC family developed by STMicroelectronics,
 	  MPUs based on ARM cortex A core
-	  U-BOOT is running in DDR and SPL support is the unsecure First Stage
-	  BootLoader (FSBL)
+	  U-BOOT is running in DDR, loaded by the First Stage BootLoader (FSBL).
+	  FSBL can be TF-A: Trusted Firmware for Cortex A, for trusted boot
+	  chain.
+	  SPL is the unsecure FSBL for the basic boot chain.
 
 config ARCH_ROCKCHIP
 	bool "Support Rockchip SoCs"