Documentation: arm: remove dead links from Marvell Berlin docs

The BG2 and BG2Q are no longer listed on Marvell's site, so the links in
the README go nowhere. The BG2Q's product brief has also been removed.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>