clk: meson: meson8b: use clk_hw_set_parent in the CPU clock notifier

Switch from clk_set_parent() to clk_hw_set_parent() now that we have a
way to configure a mux clock based on clk_hw pointers. This simplifies
the meson8b_cpu_clk_notifier_cb logic. No functional changes.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>