ARM: timer-sp: Use of_clk_get_parent_count() instead of open coding

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>