dt-bindings: fix references for iio-bindings.txt

The iio-bindings.txt was converted into two files and merged
at the dt-schema git tree at:

	https://github.com/devicetree-org/dt-schema

Yet, some documents still refer to the old file. Fix their
references, in order to point to the right URL.

Fixes: dba91f82

 ("dt-bindings:iio:iio-binding.txt Drop file as content now in dt-schema")
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4efd81eca266ca0875d3bf9d1672097444146c69.1617972339.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>