arm: mvebu: update defconfig with ATAG support when using DT

Some of the mvebu boards (mainly the development board) come with
plug-in RAM modules. This patch allows to let the bootloaders which
have no support for DTS to give the real amount of memory available on
the board.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>