rust: io: use generic read/write accessors for primitive accesses

rust: io: use generic read/write accessors for primitive accesses

By providing the required `IoLoc` implementations on `usize`, we can
leverage the generic accessors and reduce the number of unsafe blocks in
the module.

This also allows us to directly call the generic `read/write/update`
methods with primitive types, so add examples illustrating this.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260314-register-v9-6-86805b2f7e9d@nvidia.com
[ Slightly improve wording in doc-comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>