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

arch/powerpc/mm/ptdump/shared.c

@@ -30,6 +30,11 @@ static const struct flag_info flag_array[] = {
 		.val	= _PAGE_PRESENT,
 		.set	= "present",
 		.clear	= "       ",
+	}, {
+		.mask	= _PAGE_COHERENT,
+		.val	= _PAGE_COHERENT,
+		.set	= "coherent",
+		.clear	= "        ",
 	}, {
 		.mask	= _PAGE_GUARDED,
 		.val	= _PAGE_GUARDED,