mm: gmem: Add gm_dev in struct device

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7WLVX



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Add gm_dev in struct device to keep track on gmem device.

Co-developed-by: Jiangtian Feng <fengjiangtian@huawei.com>
Co-developed-by: luochunsheng <luochunsheng@huawei.com>
Signed-off-by: Chen Jun <chenjun192@huawei.com>