dt-bindings: at24: consistently document the compatible property

Current description of the compatible property for at24 is quite vague.

State explicitly that any "<manufacturer>,<model>" pair is accepted as
long as a correct fallback is used for non-atmel chips.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Rob Herring <robh@kernel.org>