soc: rockchip: power-domain: Use of_clk_get_parent_count() instead of open coding

As of_clk_get_parent_count() returns zero on failure, while
of_count_phandle_with_args() might return a negative error code, this
also fixes the issue of possibly using a negative number in the
allocation below.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>