lib: utils/fdt: Replace strcmp with strncmp

Use strncmp() instead of strcmp() in __fixup_find_domain_offset()
so that it compiles fine when linking with external firmware (such
as EDK2).

Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>