powerpc/nohash: Deduplicate _PAGE_CHG_MASK

_PAGE_CHG_MASK is identical between nohash/32 and nohash/64,
deduplicate it.

While at it, clean the #ifdef for PTE_RPN_MASK in nohash/32 as
it is already CONFIG_PPC32.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/c1a1893dbba4afb825bfa78b262f0b0e0fc3b490.1695659959.git.christophe.leroy@csgroup.eu