powerpc/mm: Add PG_dcache_clean to indicate dcache clean state

This just add a better name for PG_arch_1. No functional change in this patch.

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/20210203045812.234439-2-aneesh.kumar@linux.ibm.com