ARM: at91: fixed unresolved symbol "at91_pm_set_standby" when built without CONFIG_PM

If CONFIG_PM is not defined, then arch/arm/mach-at91/pm.c is not
compiled in.  This patch creates an inline function that does nothing
if CONFIG_PM is not defined.

Signed-off-by: Brent Taylor <motobud@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>