Include the "clk.h" header file to fix the following sparse warning:

drivers/clk/imx/clk-cpu.c:77:12: warning: symbol 'imx_clk_cpu' was not declared. Should it be static?
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>