dt-bindings: add mmio-based syscon mux controller DT bindings

This adds device tree binding documentation for mmio-based syscon
multiplexers controlled by a bitfields in a syscon register range.
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NPeter Rosin <peda@axentia.se>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/mux/mmio-mux.txt

+MMIO register bitfield-based multiplexer controller bindings
+
+Define register bitfields to be used to control multiplexers. The parent
+device tree node must be a syscon node to provide register access.
+
+Required properties:
+- compatible : "mmio-mux"
+- #mux-control-cells : <1>
+- mux-reg-masks : an array of register offset and pre-shifted bitfield mask
+                  pairs, each describing a single mux control.
+* Standard mux-controller bindings as decribed in mux-controller.txt
+
+Optional properties:
+- idle-states : if present, the state the muxes will have when idle. The
+		special state MUX_IDLE_AS_IS is the default.
+
+The multiplexer state of each multiplexer is defined as the value of the
+bitfield described by the corresponding register offset and bitfield mask pair
+in the mux-reg-masks array, accessed through the parent syscon.
+
+Example:
+
+	syscon {
+		compatible = "syscon";
+
+		mux: mux-controller {
+			compatible = "mmio-mux";
+			#mux-control-cells = <1>;
+
+			mux-reg-masks = <0x3 0x30>, /* 0: reg 0x3, bits 5:4 */
+					<0x3 0x40>, /* 1: reg 0x3, bit 6 */
+			idle-states = <MUX_IDLE_AS_IS>, <0>;
+		};
+	};
+
+	video-mux {
+		compatible = "video-mux";
+		mux-controls = <&mux 0>;
+
+		ports {
+			/* inputs 0..3 */
+			port@0 {
+				reg = <0>;
+			};
+			port@1 {
+				reg = <1>;
+			};
+			port@2 {
+				reg = <2>;
+			};
+			port@3 {
+				reg = <3>;
+			};
+
+			/* output */
+			port@4 {
+				reg = <4>;
+			};
+		};
+	};