rust: platform: Set `OF_ID_TABLE` default to `None` in `Driver` trait

Provide a default value of `None` for `Driver::OF_ID_TABLE` to simplify
driver implementations.

Drivers that do not require OpenFirmware matching no longer need to
import the `of` module or define the constant explicitly.

This reduces unnecessary boilerplate and avoids pulling in unused
dependencies.

Signed-off-by: Igor Korotin <igor.korotin.linux@gmail.com>
Link: https://lore.kernel.org/r/20250620154124.297158-1-igor.korotin.linux@gmail.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>