clk: rockchip: make clk_half_divider_ops static

clk: rockchip: make clk_half_divider_ops static

The clk_half_divider_ops is not used outside or declared
outside of drivers/clk/rockchip/clk-half-divider.c so make
it static to avoid the following warning:

drivers/clk/rockchip/clk-half-divider.c:142:22: warning: symbol 'clk_half_divider_ops' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20191017105348.8061-1-ben.dooks@codethink.co.uk
Signed-off-by: Heiko Stuebner <heiko@sntech.de>