dt-bindings:iio:adc: update adc.yaml reference

dt-bindings:iio:adc: update adc.yaml reference

Changeset b70d154d6558 ("dt-bindings:iio:adc: convert adc.txt to yaml")
renamed: Documentation/devicetree/bindings/iio/adc/adc.txt
to: Documentation/devicetree/bindings/iio/adc/adc.yaml.

Update its cross-reference accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/8e37dba8ae9099acd649bab8a1cf718caa4f3e6a.1610535350.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>