powerpc/pseries/pmem: Convert to %pOFn instead of device_node.name

In preparation to remove the node name pointer from struct
device_node, convert printf users to use the %pOFn format specifier.
pmem.c was recently added and missed the initial conversion.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>