clk: hix5hd2: Silence sparse warnings

drivers/clk/hisilicon/clk-hix5hd2.c:255:13: warning: symbol 'hix5hd2_clk_register_complex' was not declared. Should it be static?

Acked-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>