stm32mp1: add trusted boot with TF-A

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>