ARM: dts: mvebu: add NAND description to Armada 370 DB and Armada XP DB

This commit adds the Device Tree description for the 1GB NAND flash
present in the Armada 370 DB and Armada XP DB evaluation boards from
Marvell.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>