MAINTAINERS: update ti,dac7612.yaml reference

Changeset 8b74e06b ("dt-bindings:iio:dac:ti,dac7612 yaml conversion")
renamed: Documentation/devicetree/bindings/iio/dac/ti,dac7612.txt
to: Documentation/devicetree/bindings/iio/dac/ti,dac7612.yaml.

Update its cross-reference accordingly.

Fixes: 8b74e06b ("dt-bindings:iio:dac:ti,dac7612 yaml conversion")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>