nxp: imx7d-6sx: add rom_start relocation

Add the Kconfig options and use the aliased
addresses for the bootcode regions of the IMX7D
and IMX6SX SOCs to allow the Linux rproc
framework to load the irq-vectors into
the correct memory areas.

Activating this option might enlarge the bin
file if the zephyr,flash and rom_start chosen
region addresses are not matching.

It is up to the user to enable this feature
based on code location choices (OCRAM, DDR, TCM...).

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