dt-bindings:iio:adc: update motorola,cpcap-adc.yaml reference

Changeset 1ca9d1b1 ("dt-bindings:iio:adc:motorola,cpcap-adc yaml conversion")
renamed: Documentation/devicetree/bindings/iio/adc/cpcap-adc.txt
to: Documentation/devicetree/bindings/iio/adc/motorola,cpcap-adc.yaml.

Update its cross-reference accordingly.

Fixes: 1ca9d1b1

 ("dt-bindings:iio:adc:motorola,cpcap-adc yaml conversion")
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3e205e5fa701e4bc15d39d6ac1f57717df2bb4c6.1617972339.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>