MAINTAINERS: Add maintainer entry for linear ranges helper

The linear ranges helpers were refactored out of regulator core
for other drivers to enjoy. Add regulator maintainer Mark Brown as
maintainer and myself as a reviewer.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/fbbbee249c6b9df3ba63bb51ea53526b22921e84.1589866138.git.matti.vaittinen@fi.rohmeurope.com


Signed-off-by: Mark Brown <broonie@kernel.org>