ARM: at91: pm: change at91_pm_set_standby() to static

Since at91_pm_set_standby() will not be used out of the pm.c file,
change its attribute from extern to static, remove its declaration as well.

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