ARM: at91: at91rm9200: set idle and restart from rm9200_dt_device_init()

Setup arm_pm_idle and arm_pm_restart function pointers from
rm9200_dt_device_init() function to simplify the at91rm9200 initialization
process.
This same move is already done for the sam9s.

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