ARM: at91: pm: Remove at91_pm_set_standby

Merge at91_pm_set_standby() in at91_dt_ramc as this is the only callsite.
That moves it to the init section.

Acked-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>