dt-bindings: memory-controllers: Add Exynos5422 DMC device description

Add device tree bindings for a new Exynos5422 Dynamic Memory Controller
device.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>