powerpc/ptdump: Display _PAGE_READ and _PAGE_WRITE

Instead of always displaying either 'rw' or 'r ' depending on
_PAGE_RW, display 'r' or ' ' for _PAGE_READ and 'w' or ' '
for _PAGE_WRITE.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/dd8201a0f8fd87ce62a7ff2edc958b604b8ec3c0.1695659959.git.christophe.leroy@csgroup.eu