rust: regulator: add a bare minimum regulator abstraction

Add a bare minimum regulator abstraction to be used by Rust drivers.
This abstraction adds a small subset of the regulator API, which is
thought to be sufficient for the drivers we have now.

Regulators provide the power needed by many hardware blocks and thus are
likely to be needed by a lot of drivers.

It was tested on rk3588, where it was used to power up the "mali"
regulator in order to power up the GPU.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20250714-topics-tyr-regulator2-v8-1-c7ab3955d524@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>