bindings: rust: make mutators return &mut self

bindings: rust: make mutators return &mut self

In C++ bindings we can chain the mutators as they all return a reference
to the object they modify. It's a common practice to allow that in Rust
too so make all mutators that don't already do it return a mutable
reference to self.

It's also logically incorrect to make mutators borrow an immutable
reference to self. Even if that builds - as we're fiddling with C
pointers - it could change in the future. It's fine for getters but
setters should all use mutable references.

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