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.it
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>