ARM: imx: select ARM_CPU_SUSPEND for CPU_IDLE as well

The cpuidle support calls cpu_suspend(), which is compiled conditionally,
and fails to link unless something selects CONFIG_ARM_CPU_SUSPEND.

arch/arm/mach-imx/cpuidle-imx6sx.o: In function `imx6sx_enter_wait':
cpuidle-imx6sx.c:(.text+0x6c): undefined reference to `cpu_suspend'

This adds an explicit select statement here.

Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>