mm: gmem: Remove select config GMEM_DEV for gmem

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



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Driver GMEM_DEV can be enabled on demand, there is no need to select
it for gmem.

Add GPL license for gmem_dev.

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>