dt-bindings: eeprom: at25: Document device-specific compatible values

Document the recommended presence of a device-specific compatible value,
and list examples that are already in use or soon will be.
This will allow checkpatch to validate compatible values in DTS.

Update the example to match current best practices (generic node name,
specific compatible value first).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>