sw64: kvm: add qemu fw_cfg device support in sysfs

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I645LQ



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Signed-off-by: Du Yilong <duyilong@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>