ARM: at91/pm: move the standby functions to pm.c

The standby functions are now only used in pm.c, move them there.

Also, they are not inlined as a pointer to those functions is passed to the
cpuidle driver.

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