lib: utils: Disallow non-root domains from adding M-mode regions

The M-mode regions can only be added to the root domain. The non-root
domains shouldn't be able to add them from FDT.

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Anup Patel <anup@brainfault.org>