dt-bindings: rtc: DS1307 and compatibles are not trivial

Document optional properties for ds1307 and compatible RTCs

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/rtc/rtc-ds1307.txt

@@ -0,0 +1,36 @@
+Dallas DS1307 and compatible RTC
+
+Required properties:
+- compatible: should be one of:
+	"dallas,ds1307",
+	"dallas,ds1308",
+	"dallas,ds1337",
+	"dallas,ds1338",
+	"dallas,ds1339",
+	"dallas,ds1388",
+	"dallas,ds1340",
+	"dallas,ds1341",
+	"maxim,ds3231",
+	"st,m41t0",
+	"st,m41t00",
+	"microchip,mcp7940x",
+	"microchip,mcp7941x",
+	"pericom,pt7c4338",
+	"epson,rx8025",
+	"isil,isl12057"
+- reg: I2C bus address of the device
+
+Optional properties:
+- interrupt-parent: phandle for the interrupt controller.
+- interrupts: rtc alarm interrupt.
+- clock-output-names: From common clock binding to override the default output
+                      clock name
+- wakeup-source: Enables wake up of host system on alarm
+
+Example:
+	rtc1: ds1339@68 {
+		compatible = "dallas,ds1339";
+		reg = <0x68>;
+		interrupt-parent = <&gpio4>;
+		interrupts = <20 0>;
+	};