drivers: base: power: clock_ops: Use of_clk_get_parent_count()

Use of_clk_get_parent_count() instead of open coding.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>