rust: extract bitfield! macro from register!

Extract the bitfield-defining part of the `register!` macro into an
independent macro used to define bitfield types with bounds-checked
accessors.

Each field is represented as a `Bounded` of the appropriate bit width,
ensuring field values are never silently truncated.

Fields can optionally be converted to/from custom types, either fallibly
or infallibly.

Appropriate documentation is also added, and a MAINTAINERS entry created
for the new module.

Two minor fixups are also applied: the private accessors are inlined,
and a couple of missing fully qualified types in the macro are fixed.

Acked-by: Yury Norov <ynorov@nvidia.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Yury Norov <ynorov@nvidia.com>
Link: https://patch.msgid.link/20260606-bitfield-v5-1-b92188820914@nvidia.com
[ Added some more intra-doc links. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

MAINTAINERS

@@ -23422,6 +23422,13 @@ T:	git https://github.com/Rust-for-Linux/linux.git alloc-next
 F:	rust/kernel/alloc.rs
 F:	rust/kernel/alloc/
 
+RUST [BITFIELD]
+M:	Alexandre Courbot <acourbot@nvidia.com>
+R:	Yury Norov <yury.norov@gmail.com>
+L:	rust-for-linux@vger.kernel.org
+S:	Maintained
+F:	rust/kernel/bitfield.rs
+
 RUST [INTEROP]
 M:	Joel Fernandes <joelagnelf@nvidia.com>
 M:	Alexandre Courbot <acourbot@nvidia.com>

rust/kernel/lib.rs

@@ -44,6 +44,7 @@ pub mod acpi;
 pub mod alloc;
 #[cfg(CONFIG_AUXILIARY_BUS)]
 pub mod auxiliary;
+pub mod bitfield;
 pub mod bitmap;
 pub mod bits;
 #[cfg(CONFIG_BLOCK)]