powerpc/pmem: Initialize pmem device on newer hardware

With kernel now supporting new pmem flush/sync instructions, we can now
enable the kernel to initialize the device. On P10 these devices would
appear with a new compatible string. For PAPR device we have

compatible       "ibm,pmemory-v2"

and for OF pmem device we have

compatible       "pmem-region-v2"

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200701072235.223558-8-aneesh.kumar@linux.ibm.com