Add a "secure" version based on SCMI of STM32 boards. Only boards
provided by STMicroelectronics are concerned:

-STM32MP157A-DK1
-STM32MP157C-DK2
-STM32MP157C-ED1
-STM32MP157C-EV1
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>