boards: arm: stm32157c-dk2: update debug documentation
boards: arm: stm32157c-dk2: update debug documentation

Update the Debugging chapter. The stm32mp157 Cortex-M4 is acting as
a coprocessor and should not run in a standalone mode.
Indeed, the firmware has to be loaded and started by the main processor.
This is mandatory for the inter-processor communication.

For this reason user should use "west attach" instead of "west debug".

Signed-off-by: Arnaud Pouliquen <b379f9d268ae6bc79bae62bbf7d1708239982b68@foss.st.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr