ARM: dts: mediatek: Add compatible property for aquaris5

Add the missing 'compatible' property to device tree root node of

 - mt6589-aquaris5.dts

and document the new values.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>