ARM: dts: exynos: Add missing memory node for Exynos5440 boards

ARM: dts: exynos: Add missing memory node for Exynos5440 boards

The skeleton.dtsi file was removed in ARM64 for different reasons as
explained in commit ("3ebee5a2e141 arm64: dts: kill skeleton.dtsi").

These also applies to ARM and it will also allow to get rid of the
following DTC warnings in the future:

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

But these boards don't have a memory node defined, so removing the
skeleton.dtsi inclusion in exynos5440.dtsi will cause a change in
the compiled DTB. Add a dummy memory node so the compiled DTB does
not change if the skeleton.dtsi is removed from exynos5440.dtsi.

Eventually the correct starting addresses and sizes should be used
but I didn't find that information.

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