MAINTAINERS: Add IIO gain-time-scale helpers

Add myself as a maintainer for IIO light sensor helpers (helpers for
maintaining the scale while adjusting intergration time or gain) and
related Kunit tests.
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/d46414eabe8dd4cd3edb15f859f3b93cd406d9aa.1680263956.git.mazziesaccount@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

MAINTAINERS

@@ -9938,6 +9938,13 @@ F:	Documentation/ABI/testing/sysfs-bus-iio-adc-envelope-detector
 F:	Documentation/devicetree/bindings/iio/adc/envelope-detector.yaml
 F:	drivers/iio/adc/envelope-detector.c
 
+IIO LIGHT SENSOR GAIN-TIME-SCALE HELPERS
+M:	Matti Vaittinen <mazziesaccount@gmail.com>
+L:	linux-iio@vger.kernel.org
+S:	Maintained
+F:	drivers/iio/light/gain-time-scale-helper.c
+F:	drivers/iio/light/gain-time-scale-helper.h
+
 IIO MULTIPLEXER
 M:	Peter Rosin <peda@axentia.se>
 L:	linux-iio@vger.kernel.org