ARM: at91: sam9: set arm_pm_idle from sam9_dt_device_init

As all sam9 SoCs are setting arm_pm_idle to at91sam9_idle(), do it from
sam9_dt_device_init().

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
[nicolas.ferre@atmel.com: adapt patch to newer series]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>