MAINTAINERS: update dpot-dac.yaml reference

MAINTAINERS: update dpot-dac.yaml reference

Changeset 06d2ff6fe11e ("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: 06d2ff6fe11e ("dt-bindings:iio:dac:dpot-dac: yaml conversion.")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/efda999adce3332dc1b5c20a998f3824c1cc1b0f.1617279355.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>