ARM: at91: fix sam9n12 and sam9x5 arm_pm_idle

sam9n12 and sam9x5 don't set arm_pm_idle because of an oversight, fix that.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>