powerpc/pmem: Avoid the barrier in flush routines

nvdimm expect the flush routines to just mark the cache clean. The barrier
that mark the store globally visible is done in nvdimm_flush().

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/20200701072235.223558-7-aneesh.kumar@linux.ibm.com