ARM: dts: Add Samsung G2D DT bindings documentation

Added documentaion about G2D bindings.

Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>