iio: document "serialnumber" sysfs attribute

Add ABI documentation for the new "serialnumber" sysfs attribute. The
first user is the bno055 IIO driver.
Signed-off-by: Andrea Merello <andrea.merello@iit.it>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220907132205.28021-11-andrea.merello@iit.itSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -2155,3 +2155,10 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Raw (unscaled) euler angles readings. Units after
 		application of scale are deg.
+
+What:		/sys/bus/iio/devices/iio:deviceX/serialnumber
+KernelVersion:	6.1
+Contact:	linux-iio@vger.kernel.org
+Description:
+		An example format is 16-bytes, 2-digits-per-byte, HEX-string
+		representing the sensor unique ID number.