dt-bindings: extcon: Add support for SM5502 MUIC device

This patch add documentation for binding of SM5502 MUIC (Micro-USB Interface
Controller) device which is using EXTCON subsystem. The SM5502 MUIC device
can detect various external accessories when external accessories is attached
or detached.
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

Documentation/devicetree/bindings/extcon/extcon-sm5502.txt

+
+* SM5502 MUIC (Micro-USB Interface Controller) device
+
+The Silicon Mitus SM5502 is a MUIC (Micro-USB Interface Controller) device
+which can detect the state of external accessory when external accessory is
+attached or detached and button is pressed or released. It is interfaced to
+the host controller using an I2C interface.
+
+Required properties:
+- compatible: Should be "siliconmitus,sm5502-muic"
+- reg: Specifies the I2C slave address of the MUIC block. It should be 0x25
+- interrupt-parent: Specifies the phandle of the interrupt controller to which
+  the interrupts from sm5502 are delivered to.
+- interrupts: Interrupt specifiers for detection interrupt sources.
+
+Example:
+
+	sm5502@25 {
+		compatible = "siliconmitus,sm5502-muic";
+		interrupt-parent = <&gpx1>;
+		interrupts = <5 0>;
+		reg = <0x25>;
+	};