ARM: mach-imx: Select ARM_CPU_SUSPEND at ARCH_MXC level

Instead of selecting ARM_CPU_SUSPEND only for mx6, we can select it for
all SoCs from the ARCH_MXC family.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>