ARM: dts: mvebu: a385-db-ap: add default partition description for NAND

The Armada 385 Access Point Development board contains NAND FLSH which is
already enabled in existing dts. Nevertheless the default partition
description was missing.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>