MAINTAINERS: add rust bindings entry for bitmap API

MAINTAINERS: add rust bindings entry for bitmap API

This entry enumerates bitmap and related APIs listed in BITMAP API entry
that rust requires but cannot use directly (i.e. inlined functions and
macros).

The "Rust kernel policy" (https://rust-for-linux.com/rust-kernel-policy)
document describes the special status of rust support:

  "Exceptionally, for Rust, a subsystem may allow to temporarily
   break Rust code."

Accordingly, the following policy applies to all interfaces under the
BITMAP API entry that are used in rust codebase, including those not
listed explicitly here.

Bitmap developers do their best to keep the API stable. When API or
user-visible behavior needs to be changed such that it breaks rust,
bitmap and rust developers collaborate as follows:
 - bitmap developers don't consider rust bindings as a blocker for the
   API change;
 - bindings maintainer (me) makes sure that kernel build doesn't break
   with CONFIG_RUST=y. This implies fixes in the binding layer, but not
   in rust codebase;
 - rust developers adopt new version of API in their codebase and remove
   unused bindings timely.

CC: Danilo Krummrich <dakr@redhat.com>
CC: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>