clk: qcom: cpu-8996: don't store parents in clk_cpu_8996_pmux

Don't store pointers to parents in struct clk_cpu_8996_pmux. Instead use
clk_hw_get_parent_by_index to fetch them.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220714100351.1834711-6-dmitry.baryshkov@linaro.org