clk: Stop forwarding clk_rate_requests to the parent

If the clock cannot modify its rate and has CLK_SET_RATE_PARENT,
clk_mux_determine_rate_flags() and clk_core_round_rate_nolock() will
call clk_core_round_rate_nolock() with its parent clock but use the
request of the child node either directly (clk_core_round_rate_nolock())
or by copying it (clk_mux_determine_rate_flags()).

Both cases are problematic since the parent will now have a request with
the best parent fields of the child (so pointing to itself) and the
boundaries of the child as well.

clk_core_round_rate_nolock() is even worse since we would directly
modify the caller structure if the parent was ever to modify its own
parent or its parent rate, then returning to the caller a best parent
that isn't a parent of the clock we just called clk_determine_rate()
onto.

Let's create a new function that will create a new request to forward to
the parent, clk_core_forward_rate_req() and update the relevant call
sites to that new function.

Let's also add a test to make sure we avoid regressions there.

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>