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>