ARM: imx: build CPU suspend code only when needed

The ARM CPU suspend function has its own configuration symbol,
which we need to use for conditionalizing any code calling into
it as well.

arch/arm/mach-imx/built-in.o: In function `v7_cpu_resume':
/git/arm-soc/arch/arm/mach-imx/headsmp.S:57: undefined
 reference to `cpu_resume'

Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>