imx_clk_composite: Fix up clock selection

[thj]

Summary

Fix up the clock selection logic so we are more likely to find a best
parent and update the clock register when we have a new clock.

• imx_clk_composite_find_best returns 0 on sucess, uninvert the test so we don't skip a successful look up.
• When we do find a new best parent write out the best divisors rather than the last tested ones.
• Finally, update the clock register to make the change.

If we don't find a new best_parent this still skips doing anything, @mmel I'm
not sure if returnin an error is best here or trying to do a nop update of the
clock and returning the freq in fout.

Test Plan

This doesn't obviously break anything on my test hardware, but it also doesn't
help with any of my current hardware issues which might be clock related. It
has exposed some missing clocks in the imx8mp_ccm driver, which I have a fix
for in my queue.