arch: arm: rom_start relocation configuration

In order to support Linux rproc loading, some SOCs require
the boot-vector and irq-vectors to be placed into a defined
memory area for the mcu to boot.

This is necessary for NXP's IMX SOCs for instance but
can be leveraged by other SOCs that have multiple
zephyr,flash choices.

Signed-off-by: Jérémy LOCHE - MAKEEN Energy <jlh@makeenenergy.com>