dt-bindings: Relocate DDR bindings

Move DDR bindings to memory-controllers directory to make them more
discoverable.

Suggested-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211006224659.21434-2-digetx@gmail.com


[krzysztof: Correct path in lpddr3.txt and samsung,exynos5422-dmc.yaml]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>