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