MAINTAINERS: fix location of ina2xx.txt device tree file

This file got moved and merged, causing the old reference to
not exist anymore. Fix it.

Fixes: 6e24d205 ("hwmon: ina209: move binding docs to proper place")
Fixes: 62bc9f15 ("dt-bindings: merge ina209 binding into ina2xx binding")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>