dt-bindings: at24: add ST M24C64-D Additional Write lockable page

The ST M24C64-D behaves as a regular M24C64, except for the -D variant
which uses up another I2C address for Additional Write lockable page.
This page is 32 Bytes long and can contain additional data. Document
compatible string for it, so users can describe that page in DT. Note
that users still have to describe the main M24C64 area separately as
that is on separate I2C address from this page.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>