rust: clk: Add helpers for Rust code

rust: clk: Add helpers for Rust code

Non-trivial C macros and inlined C functions cannot be used directly
in the Rust code and are used via functions ("helpers") that wrap
those so that they can be called from Rust.

In order to prepare for adding Rust abstractions for the clock APIs,
add clock helpers required by the Rust implementation.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>