nvdimm: make nd_class constant

Now that the driver core allows for struct class to be in read-only
memory, it is possible to make all 'class' structures be declared at
build time. Move the class to a 'static const' declaration and register
it rather than dynamically create it."

Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: nvdimm@lists.linux.dev
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/2024061041-grandkid-coherence-19b0@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>