dt-bindings: extcon: Add support for Richtek RT8973A MUIC device

This patch add documentation for binding of Richtek RT8973A (Micro USB Switch)
device which is using EXTCON subsystem. The RT8973A device can detect various
external accessories when external accessories is attached or detached.
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>

Documentation/devicetree/bindings/extcon/extcon-rt8973a.txt

+
+* Richtek RT8973A - Micro USB Switch device
+
+The Richtek RT8973A is Micro USB Switch with OVP and I2C interface. The RT8973A
+is a USB port accessory detector and switch that is optimized to protect low
+voltage system from abnormal high input voltage (up to 28V) and supports high
+speed USB operation. Also, RT8973A support 'auto-configuration' mode.
+If auto-configuration mode is enabled, RT8973A would control internal h/w patch
+for USB D-/D+ switching.
+
+Required properties:
+- compatible: Should be "richtek,rt8973a-muic"
+- reg: Specifies the I2C slave address of the MUIC block. It should be 0x14
+- interrupt-parent: Specifies the phandle of the interrupt controller to which
+  the interrupts from rt8973a are delivered to.
+- interrupts: Interrupt specifiers for detection interrupt sources.
+
+Example:
+
+	rt8973a@14 {
+		compatible = "richtek,rt8973a-muic";
+		interrupt-parent = <&gpx1>;
+		interrupts = <5 0>;
+		reg = <0x14>;
+	};