ARM: imx6: build suspend-imx6.o with CONFIG_SOC_IMX6

Even when CONFIG_SUSPEND is enabled, it makes no sense to build
suspend-imx6.o if none of i.MX6 support is built in.  Let's build
suspend-imx6.o only when both CONFIG_SUSPEND and CONFIG_SOC_IMX6 are
enabled.

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