boards: nxp: frdm_mcxn947: update dual-core memory docs

Update the dual-core memory configuration documentation to use
symbolic references to flash regions and partitions instead of
hardcoded addresses. This makes the documentation more maintainable.

The updated documentation correctly reflects that CPU0 has access to
the full flash memory including the bootloader region, while CPU1 is
restricted to its dedicated slot1_partition region.

Signed-off-by: Tomas Galbicka <tomas.galbicka@nxp.com>