clk: add needed include and declaration to include/clk.h

This header uses ulong, so it needs to include <linux/types.h>.
Likewise, "struct udevice" must be declared before it is used.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>