ARM: dts: exynos: Add DSI node on Exynos5250

Add common part of DSI node for Exynos5250 platforms and a required
mipi-phy node.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>