ARM: imx6: move v7_cpu_resume() into suspend-imx6.S

The suspend-imx6.S is introduced recently for suspend low-level assembly
code.  Since function v7_cpu_resume() is only used by suspend support,
it makes sense to move the function into suspend-imx6.S, and control the
build of the file with CONFIG_SUSPEND option.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>