imx clk: Skip reconfiguring the clock if we don't have a new best_parent

imx_clk_composite_find_best_parent can fail, when it does for all the
clocks checked we attempt to configure an uninitialized best_parent
leading to a panic.

Initialize best_parent and skip reconfiguring the clock if we don't find
a new best_parent to use.

Reviewed By: manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46662