dt-bindings:iio:dac: update microchip,mcp4725.yaml reference

Changeset 6ced946a ("dt-bindings:iio:dac:microchip,mcp4725 yaml conversion")
renamed: Documentation/devicetree/bindings/iio/dac/mcp4725.txt
to: Documentation/devicetree/bindings/iio/dac/microchip,mcp4725.yaml.

Update its cross-reference accordingly.

Fixes: 6ced946a ("dt-bindings:iio:dac:microchip,mcp4725 yaml conversion")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>