boards: arm: stm32h573 disco kit move partition to ext flash

Add the XSPI 2 which is an octoSPI connection to a octo NOR flash
mx25lm51245 on the stm32h573i_dk disco kit.
Define the partition on the 64MBytes space of the external flash.
Use the STM32Cube programmer to flash with the corresponding
external loader for XiP mode.
Removing usb_device will avoid test feature for that board.

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