nfit, tools/testing/nvdimm: unify common init for acpi_nfit_desc

nfit, tools/testing/nvdimm: unify common init for acpi_nfit_desc

The nvdimm unit test infrastructure performs its own initialization of
an acpi_nfit_desc to specify test overrides over the native
implementation.  Make it clear which attributes and operations it is
overriding by re-using acpi_nfit_init_desc() as a common starting point.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>