ARM: dts: stm32: Manage security diversity for STM32M15x SOCs

This commit creates a new file to manage security diversity on STM32MP15x
SOCs. On STM32MP15xY, "Y" gives information:
 -Y = A means no cryp IP and no secure boot.
 -Y = C means cryp IP + secure boot.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>