ARM: configs: enable Marvell Armada 39x in multi_v7_defconfig

Following the introduction of the Marvell Armada 39x support, let's
enable this support by default in multi_v7_defconfig.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: <arm@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>