clk: rockchip: Mark rockchip_fractional_approximation static

Silence the sparse warning

clk/rockchip/clk.c:172:6: warning: symbol 'rockchip_fractional_approximation' was not declared. Should it be static?

Cc: Elaine Zhang <zhangqing@rock-chips.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/rockchip/clk.c

@@ -169,7 +169,7 @@ static int rockchip_clk_frac_notifier_cb(struct notifier_block *nb,
  * fractional divider must set that denominator is 20 times larger than
  * numerator to generate precise clock frequency.
  */
-void rockchip_fractional_approximation(struct clk_hw *hw,
+static void rockchip_fractional_approximation(struct clk_hw *hw,
 		unsigned long rate, unsigned long *parent_rate,
 		unsigned long *m, unsigned long *n)
 {