dt-bindings: clk: samsung: Document the DMC domain of Exynos3250 CMU

Document the new compatible for clock in DMC (Dynamic Memory
Controller) domain of Exynos3250 Clock Management Unit (CMU).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>