pmem: compile pmem legacy device while libnvdimm as a module

openeuler inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5BHF6


CVE: NA

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When we use nvdimms as pmem block devices, we found no pmem devices created,
because of pmem_legacy_device not registed.

It would be invoked in pmem_legacy_device.c as device_initcall.
But when only CONFIG_PMEM_LEGACY_DEVICE set and CONFIG_LIBNVDIMM set to m,
drivers/nvdimm will not be compiled at obj-y.

So we need compile drivers/nvdimm if only CONFIG_PMEM_LEGACY_DEVICE set.

Signed-off-by: snoweay <snoweay@163.com>