ARM: add device-tree SoC bindings for Axis Artpec-6

This adds device tree bindings for the Artpec-6 SoC.

Signed-off-by: Lars Persson <larper@axis.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>