ARM: add Marvell Berlin SoC familiy to Marvell doc

This adds known facts and rumors about the Marvell Berlin (88DE3xxx) SoC
family to the Marvell SoC documentation.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Reviewed-by: Jason Cooper <jason@lakedaemon.net>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>