doc: bindings: fix bad reference to ARM CPU bindings

The primecell.txt and cpus.txt files were converted into YAML. This
patch updates old references with new ones.

Fixes: d3c207ee ("dt-bindings: arm: Convert primecell binding to json-schema")
Fixes: 672951cb

 ("dt-bindings: arm: Convert cpu binding to json-schema")
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Rob Herring <robh@kernel.org>