boot: zephyr: boards config of the stm32h573 disco kit

Define the stm32h573 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
The zephyr, chosen zephyr,flash = &flash0; is in the board dts.
The zephyr,code-partition = &boot_partition; is given by the app.overlay

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