spapr: nvdimm: Fix the persistent-memory root node name in device tree

spapr: nvdimm: Fix the persistent-memory root node name in device tree

The FDT code is adding the pmem root node by name "persistent-memory"
which should have been "ibm,persistent-memory".

The linux fetches the device tree nodes by type and it has been working
correctly as the type is correct. If someone searches by its intended
name it would fail, so fix that.

Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Message-Id: <162204278956.219.9061511386011411578.stgit@cc493db1e665>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>