clk: Introduce clk_hw_init_rate_request()

Some drivers (at91, imx, qcom) use __clk_determine_rate directly, and
thus will need to initialise a clk_rate_request structure.

However, drivers don't have access to the function that the core uses to
initialize that structure which could prove to be useful.

Let's create a function for clock providers that will initialize a
clk_rate_request for a given clk_hw pointer and a rate.

Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com> # imx8mp
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com> # exynos4210, meson g12b
Signed-off-by: Maxime Ripard <maxime@cerno.tech>