ARM: dts: Add FIMD DT binding Documentation

Add DT binding documentation for the FIMD IP block found in Samsung SoCs.

Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>