efi: Make efi_find_mirror() public

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I4PM01


CVE: NA

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Commit b05b9f5f ("x86, mirror: x86 enabling - find mirrored memory
ranges") introduce the efi_find_mirror function on x86. In order to reuse
the API we make it public in preparation for arm64 to support mirrord
memory.

Co-developed-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>