boot: zephyr: boards config of the stm32h7rs target boards

Define the stm32h7s7 board configuration to set
the STM32 XIP mode that will enable the MemoryMapped mode.
Gives larger sector size for the external NOR flash
See Kconfig of the stm32 soc. Choose the mcu_boot partition
where to build ../bootloader/mcuboot/boot/zephyr

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