clk: mediatek: refactor parent rate lookup functions

clk: mediatek: refactor parent rate lookup functions

Refactor duplicate parent rate lookup code into a common function.

Instead of relying on rules like X is always the parent of Y, we use
the driver ops pointer to make sure we are actually getting the correct
parent clock device. This allows the same function to be called from
different clock types and will allow future chip-specific clock drivers
to not have to follow the rules as strictly.

Reviewed-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20260303-mtk-mt8189-clocks-v4-2-ee85f8dd2f0d@baylibre.com
Signed-off-by: David Lechner <dlechner@baylibre.com>