clk: mediatek: fix mux clocks with mapped parent IDs

clk: mediatek: fix mux clocks with mapped parent IDs

Pass the unmapped parent ID when setting parent for mux clocks.

For technical reasons, some Mediatek clock driver have a mapping between
the clock IDs used in the devicetree and ID used in the generic clock
framework.

The mtk_clk_mux_set_parent() function is comparing the passed mapped
parent ID against the unmapped IDs in the chip-specific data structures.
Before this change, we were passing the mapped parent ID. When there is
a mapping, this resulted in buggy behavior (usually just incorrectly
failing to find a match and returning an error). We need to pass the
unmapped ID of the parent clock instead for the matching to work
correctly.

Since the reverse lookup is a bit verbose, a helper function is added to
keep the code clean.

Fixes: b1358915728b ("clk: mediatek: add of_xlate ops")
Signed-off-by: David Lechner <dlechner@baylibre.com>