Documentation: arm: update homepage URLs for Marvell Berlin SoCs

Marvell has renamed their Berlin family from "Digital Entertainment" to
"Multimedia Solutions." There aren't proper redirects set up for
device-specific pages, so update the URLs accordingly.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>