config: Add default value for CONFIG_SERIAL_ATTACHED_MBIGEN

ascend inclusion
category: bugfix
Bugzilla: N/A
CVE: N/A

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Signed-off-by: Xu Qiang <xuqiang36@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>