dt-bindings: add documentation for Mediatek SoC

This adds a DT binding documentation for the MT6589 SoC from Mediatek.
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/mediatek.txt

+Mediatek MT6589 Platforms Device Tree Bindings
+
+Boards with a SoC of the Mediatek MT6589 shall have the following property:
+
+Required root node property:
+
+compatible: must contain "mediatek,mt6589"
+