boards: st: stm32h7s3l8 nucleo board for XiP on external flash

Add the memory attributes to XiP in MemoryMapped mode on the
external octo-flash of the nucleo_h7s3l8 board.
Use the STM32Cube programmer to flash with the corresponding
external loader for XiP mode.

Signed-off-by: Francois Ramu <francois.ramu@st.com>