etmem: add ETMEM feature CONFIG to mm/Kconfig

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8T1MB?from=project-issue


CVE: NA

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etmem, the memory vertical expansion technology,
uses DRAM and high-performance storage new media to form multi-level
memory storage.

Signed-off-by: Yuchen Tang <tangyuchen5@huawei.com>