clk: divider: Make divider_round_rate take the parent clock

So far, divider_round_rate only considers the parent clock returned by
clk_hw_get_parent.

This works fine on clocks that have a single parents, this doesn't work on
muxes, since we will only consider the first parent, while other parents
may totally be able to provide a better combination.

Clocks in that case cannot use divider_round_rate, so would have to come up
with a very similar logic to work around it. Instead of having to do
something like this, and duplicate that logic everywhere, create a
divider_round_rate parent to allow caller to give an additional parameter
for the parent clock to consider.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>