Input: snvs_pwrkey - use dev_pm_set_wake_irq() to simplify code

With calling dev_pm_set_wake_irq() to set SNVS ON/OFF button
as wakeup source for suspend, generic wake irq mechanism
will automatically enable it as wakeup source when suspend,
then the enable_irq_wake()/disable_irq_wake() can be removed
in suspend/resume callback, it simplifies the code.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>