rust: io: provide Mmio relaxed ops through a wrapper type

rust: io: provide Mmio relaxed ops through a wrapper type

Relaxed I/O accessors for `Mmio` are currently implemented as an extra
set of methods that mirror the ones defined in `Io`, but with the
`_relaxed` suffix.

This makes these methods impossible to use with generic code, which is a
highly plausible proposition now that we have the `Io` trait.

Address this by adding a new `RelaxedMmio` wrapper type for `Mmio` that
provides its own `IoCapable` implementations relying on the relaxed C
accessors. This makes it possible to use relaxed operations on a `Mmio`
simply by wrapping it, and to use `RelaxedMmio` in code generic against
`Io`.

Acked-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260206-io-v2-3-71dea20a06e6@nvidia.com
[ Use kernel import style in examples. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>