mfd: dt: Add bindings for the Aspeed SoC Display Controller (GFX)

The Aspeed SoC Display Controller is presented as a syscon device to
arbitrate access by display and pinmux drivers. Video pinmux
configuration on fifth generation SoCs depends on bits in both the
System Control Unit and the Display Controller.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/aspeed-gfx.txt

+* Device tree bindings for Aspeed SoC Display Controller (GFX)
+
+The Aspeed SoC Display Controller primarily does as its name suggests, but also
+participates in pinmux requests on the g5 SoCs. It is therefore considered a
+syscon device.
+
+Required properties:
+- compatible:		"aspeed,ast2500-gfx", "syscon"
+- reg:			contains offset/length value of the GFX memory
+			region.
+
+Example:
+
+gfx: display@1e6e6000 {
+	compatible = "aspeed,ast2500-gfx", "syscon";
+	reg = <0x1e6e6000 0x1000>;
+};