add ETMEM feature CONFIG to mm/Kconfig

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7RO5Q
Reference: https://gitee.com/openeuler/kernel/commit/59abec29c6ca44301d034590d021d17cdd84e7cb



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

The etmem feature was introduced in the previous commit
(aa7f1d22),but only the config
options for the etmem_swap and etmem_scan modules were added,
and the config options for the etmem feature were not added,
so in this commit, the CONFIG_ETMEM option for
the etmem feature was added

Signed-off-by: liubo <liubo254@huawei.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>