bindings: rust: expose v2.1 features as flag

bindings: rust: expose v2.1 features as flag

v2.1 provided a getter to read the chip name from a request.

This adds v2_1 as feature in order to raise the minimum requested version
to v2.1 for exposing the new API.

This is identical to the concept patch that was posted [1] when this
feature flag mechanism was proposed. Only the commit message was
reworded.

[1] https://lore.kernel.org/all/20231006-b4-bindings-old-version-fix-v1-2-a65f431afb97@linaro.org/

Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>