powerpc/mm/book3s64: Use 256M as the upper limit with coherent device memory attached

Commit 4d157211 ("powerpc/mm: Cleanup memory block size probing")
used 256MB as the memory block size when we have
ibm,coherent-device-memory device tree node present. Instead of
returning with 256MB memory block size, continue to check the rest of
the memory regions and make sure we can still map them using a 256MB
memory block size.

Fixes: 4d157211 ("powerpc/mm: Cleanup memory block size probing")
Signed-off-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230828074658.59553-2-aneesh.kumar@linux.ibm.com