ARM: mvebu: enable Armada 39x in mvebu_v7_defconfig

Following the introduction of the Marvell Armada 39x support, let's
enabled it by default in mvebu_v7_defconfig.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>