powerpc: Deduplicate prototypes of ptep_set_access_flags() and phys_mem_access_prot()

Prototypes of ptep_set_access_flags() and phys_mem_access_prot() are identical
for book3s and nohash.

Deduplicate them.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b846832cce842a2852615b7356937fb9507e436d.1695659959.git.christophe.leroy@csgroup.eu