i2c: designware_i2c: Include clk.h in the header file

We use struct clk here so really should include this header file to avoid
build errors. Also switch the order of clk.h in the C file to match the
required code style.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-by: Jun Chen <ptchentw@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>