rust: device: implement device context for Device

Analogous to bus specific device, implement the DeviceContext generic
for generic devices.

This is used for APIs that work with generic devices (such as Devres) to
evaluate the device's context.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250413173758.12068-4-dakr@kernel.org


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