dt-bindings: add document for analogix display port driver

Analogix dp driver is split from exynos dp driver, so we just
make an copy of exynos_dp.txt, and then simplify exynos_dp.txt

Beside update some exynos dtsi file with the latest change
according to the devicetree binding documents.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>