ARM: dts: exynos: Add missing unit name to memory nodes in Exynos4 DTS

This patch fixes the following DTC warnings:

"Node /memory has a reg or ranges property, but no unit name"

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>