rust: device: add drvdata accessors

rust: device: add drvdata accessors

Implement generic accessors for the private data of a driver bound to a
device.

Those accessors should be used by bus abstractions from their
corresponding core callbacks, such as probe(), remove(), etc.

Implementing them for device::CoreInternal guarantees that driver's can't
interfere with the logic implemented by the bus abstraction.

Acked-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250621195118.124245-3-dakr@kernel.org
[ Improve safety comment as proposed by Benno. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>