powerpc/ptdump: Add _PAGE_COHERENT flag

For platforms using shared.c (4xx, Book3e, Book3s/32), also handle the
_PAGE_COHERENT flag which corresponds to the M bit of the WIMG flags.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Make it more verbose, use "coherent" rather than "m"]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/324c3d860717e8e91fca3bb6c0f8b23e1644a404.1589866984.git.christophe.leroy@csgroup.eu