soc: nxp: imxrt118x: Change to use __rom_region_start symbol

The boot container was using a non-standard symbol name
`__rom_start_address` for the ROM start address. This symbol
is not defined when build with `-DCONFIG_CMAKE_LINKER_GENERATOR=y`.

This commit use the symbol `__rom_region_start` which is
available for both cases.

Signed-off-by: Jason Yu <zejiang.yu@nxp.com>