ARM: mach-imx: clk-gate2: Include "clk.h"

Fix the following sparse warning:

arch/arm/mach-imx/clk-gate2.c:86:12: warning: symbol 'clk_register_gate2' 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>