rtc: isl1219: add device tree documentation

The devicetree documentation for the ISL1219 device tree
binding is added with a short example. It is not a trivial
device, because it supports two interrupt sources.
Signed-off-by: Denis Osterland <Denis.Osterland@diehl.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/isil,isl1219.txt

+Intersil ISL1219 I2C RTC/Alarm chip with event in
+
+ISL1219 has additional pins EVIN and #EVDET for tamper detection.
+
+Required properties supported by the device:
+
+ - "compatible": must be "isil,isl1219"
+ - "reg": I2C bus address of the device
+
+Optional properties:
+
+ - "interrupt-names": list which may contains "irq" and "evdet"
+ - "interrupts": list of interrupts for "irq" and "evdet"
+ - "isil,ev-evienb": if present EV.EVIENB bit is set to the specified
+                     value for proper operation.
+
+
+Example isl1219 node with #IRQ pin connected to SoC gpio1 pin12
+ and #EVDET pin connected to SoC gpio2 pin 24:
+
+	isl1219: rtc@68 {
+		compatible = "isil,isl1219";
+		reg = <0x68>;
+		interrupt-names = "irq", "evdet";
+		interrupts-extended = <&gpio1 12 IRQ_TYPE_EDGE_FALLING>,
+			<&gpio2 24 IRQ_TYPE_EDGE_FALLING>;
+		isil,ev-evienb = <1>;
+	};
+