dt-bindings: trivial-devices: Drop incorrect and duplicate at24 compatibles

"at,24c08" does not have a correct vendor prefix. The correct compatible
string would be "atmel,24c08" which is already documented in at24.yaml.
It is also unused anywhere, so just drop it.

"st,24c256" is already documented in at24.yaml, so drop it as well.

Link: https://lore.kernel.org/r/20240903-dt-trivial-devices-v1-1-ad684c754b9c@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>