powerpc/pseries/scm: Mark the region volatile if cache flush not required

The device tree node is documented as below:

  “ibm,cache-flush-required”:
  property name indicates Cache Flush Required for this Persistent Memory Segment to persist memory
  prop-encoded-array: None, this is a name only property.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>