dt-bindings: eeprom: rename to at24.txt

This binding documentation is for the at24 driver, so the filename
should reflect it. This avoids confusion because we also have an
"eeprom" driver in Linux but it doesn't support DT even.
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NBartosz Golaszewski <brgl@bgdev.pl>