clk: hi6220: fix missing clk.h include

Fix the warning from missing "clk.h" include which
defines hi6220_register_clkdiv() function.

drivers/clk/hisilicon/clkdivider-hi6220.c:102:12: warning: symbol 'hi6220_register_clkdiv' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>