libnvdimm, of_pmem: workaround OF_NUMA=n build error

Stephen reports that an x86 allmodconfig build fails to build the
of_pmem driver due to a missing definition of of_node_to_nid(). That
helper is currently only exported in the OF_NUMA=y case. In other cases,
ppc and sparc, it is a weak symbol, and outside of those platforms it is
a static inline.

Until an OF_NUMA=n configuration can reliably support usage of
of_node_to_nid() in modules across architectures, mark this driver as
'bool' instead of 'tristate'.

Cc: Rob Herring <robh@kernel.org>
Cc: Oliver O'Halloran <oohall@gmail.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>