video: exynos_dp: device tree documentation

Add documentation for the DT bindings in exynos display port driver.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>