powerpc/nohash: Move 8xx version of pte_update() into pte-8xx.h

No point in having 8xx special pte_update() in common header,
move it into pte-8xx.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/17e209b1a1a43ed219e9e1f2947ec594ed4f9394.1695659959.git.christophe.leroy@csgroup.eu