clk: Store clk_core for clk_rate_request

The struct clk_rate_request is meant to store the context around a rate
request such as the parent, boundaries, and so on.

However, it doesn't store the clock the rate request is submitted to,
which makes debugging difficult.

Let's add a pointer to the relevant clk_core instance in order to
improve the debugging of rate requests in a subsequent patch.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20221018-clk-rate-request-tracing-v2-1-5170b363c413@cerno.tech
Signed-off-by: Stephen Boyd <sboyd@kernel.org>