dt-bindings: rtc: sd3078: add device tree documentation

The devicetree documentation for the SD3078 device tree.
Signed-off-by: Dianlong Li <long17.cool@163.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/rtc.txt

@@ -62,3 +62,4 @@ ricoh,rs5c372b		I2C bus SERIAL INTERFACE REAL-TIME CLOCK IC
 ricoh,rv5c386		I2C bus SERIAL INTERFACE REAL-TIME CLOCK IC
 ricoh,rv5c387a		I2C bus SERIAL INTERFACE REAL-TIME CLOCK IC
 sii,s35390a		2-wire CMOS real-time clock
+whwave,sd3078		I2C bus SERIAL INTERFACE REAL-TIME CLOCK IC