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>

Documentation/devicetree/bindings/arm/axis.txt

+Axis Communications AB
+ARTPEC series SoC Device Tree Bindings
+
+ARTPEC-6 ARM SoC
+================
+
+Required root node properties:
+- compatible = "axis,artpec6";
+
+ARTPEC-6 System Controller
+--------------------------
+
+The ARTPEC-6 has a system controller with mixed functions controlling DMA, PCIe
+and resets.
+
+Required properties:
+- compatible: "axis,artpec6-syscon", "syscon"
+- reg: Address and length of the register bank.
+
+Example:
+	syscon {
+		compatible = "axis,artpec6-syscon", "syscon";
+		reg = <0xf8000000 0x48>;
+	};
+
+ARTPEC-6 Development board:
+---------------------------
+Required root node properties:
+- compatible = "axis,artpec6-dev-board", "axis,artpec6";