ARM: imx6: cpuidle: Use raw_spinlock_t

The idle call back is invoked with disabled interrupts and requires
raw_spinlock_t locks to work.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>