powerpc/pmem: Convert to EXPORT_SYMBOL_GPL

All other architecture export this as GPL symbol

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/20191202064018.155083-1-aneesh.kumar@linux.ibm.com