dt-bindings: at24: convert the binding document to yaml

Convert the binding document for at24 EEPROMs from txt to yaml. The
compatible property uses a regex pattern to address all the possible
combinations of "vendor,model" strings.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[robh: rework compatible schema, fix missing allOf for $ref, fix errors in example]
Signed-off-by: Rob Herring <robh@kernel.org>
[Bartosz: added comments explaining the compatible property]
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>