dt-bindings: at24: add Renesas R1EX24128

Document the compatible value for the Renesas R1EX24128ASAS0A two-wire
serial interface EEPROM, so it can be used in DTS files without causing
checkpatch warnings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>