soc: arm: Add CONFIG_STM32H7_BOOT_M4_AT_INIT

This new Kconfig option lets the developer configure if the Cortex M4
core should be force-booted during M7 init independent of the BCM4
option byte.

This allows to disable this default behaviour via Kconfig which was not
possible so far.

Signed-off-by: Jan Krautmacher <jan@krautmacher.org>