ARM: dts: rockchip: Add missing unit name to memory nodes in rk3xxx boards

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: Heiko Stuebner <heiko@sntech.de>