powerpc/nohash: Add _PAGE_WRITE to supplement _PAGE_RW

powerpc/nohash: Add _PAGE_WRITE to supplement _PAGE_RW

Several places, _PAGE_RW maps to write permission and don't
always imply read. To make it more clear, do as book3s/64 in
commit c7d54842deb1 ("powerpc/mm: Use _PAGE_READ to indicate
Read access") and use _PAGE_WRITE when more relevant.

For the time being _PAGE_WRITE is equivalent to _PAGE_RW but that
will change when _PAGE_READ gets added in following patches.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/1f79b88db54d030ada776dc9845e0e88345bfc28.1695659959.git.christophe.leroy@csgroup.eu