MAINTAINERS: update dpot-dac.yaml reference

Changeset 06d2ff6f ("dt-bindings:iio:dac:dpot-dac: yaml conversion.")
renamed: Documentation/devicetree/bindings/iio/dac/dpot-dac.txt
to: Documentation/devicetree/bindings/iio/dac/dpot-dac.yaml.

Update its cross-reference accordingly.

Fixes: 06d2ff6f ("dt-bindings:iio:dac:dpot-dac: yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>