clk: Change clk_core_init_rate_req prototype

The expectation is that a clk_rate_request structure is supposed to be
initialized using clk_core_init_rate_req(), yet the rate we want to
request still needs to be set by hand.

Let's just pass the rate as a function argument so that callers don't
have any extra work to do.

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>